My short answer to this question is that I would recommend sending your magneto to Tony Cooper for rebuilding with a request that he should fit the later end plate, assuming he has one available.
The long answer starts with me buying my first Square Four, a 1948 4G. and putting it on the road in 1988. Although I persuaded the magneto to spark reliably, long trips showed the familiar problem of misfiring once the magneto got fully hot. The problem was mitigated somewhat by a spate of rear wheel punctures (another story) but sitting at the roadside changing inner tubes did allow the magneto to cool down and spark properly once more. Having bought a copy of Square Forum, I converted to coil ignition according to the instructions. To my surprise, the misfiring when hot problem continued. The problem was fully documented in Cheval of June 1990 - Page 20 - available on the website now , courtesy of the excellent efforts of Pete Collins - thank you Pete. Although several issues are listed in that article, the major one is that unless you isolate the magneto condenser, it will remain in circuit and if defective (which they usually are) will continue to give the misfiring problem after you have converted to coil and fitted a (good) external condenser.
Although I haven't tried it lately, I think the only way to isolate the magneto condenser involves removing it from the armature which in turn requires removal of the armature windings. This seemed like a good idea anyway, to avoid the problems of jamming due to melted shellac and provides the opportunity to use the old HT pickup as a second pickup from the points to the coil by fitting a wire between the old slip ring and the centre contact on the points plate. I used for this purpose a length of the thick primary winding copper wire which I had stripped from the armature. In time, this proved a poor choice.
By 1991, the Square was performing well and reliably, to the extent that I was able to complete my first London - Edinburgh run on it (with sidecar) that year. It continued to perform well and reliably for a further couple of years until I rebuilt it in 1994/5 and fitted a different sidecar. This time, I used a small battery on the bike instead of a large one in the sidecar boot. The bike had been on 12 volt electrics ever since the coil conversion.
In Spring 1997, I managed to get my 1936 OHC Four on the road. As it had the same magneto (apparently) as the 1948 Square, I fitted the coil conversion (and 12 volt electrics) straight away. I had multiple problems of misfiring. Eventually, I came to the conclusion that the high revving OHC four simply got the points (which run at engine speed on a Square so at twice the speed of (say a Triumph) twin) to the stage when they would not open and close correctly at moderate revs and above. This tied in with advice from none other than Titch Allen who told me that after about 10,000 miles the only thing to do with a Square Four magneto was to mend it with a new one. This was his experience from the time they were made. A huge amount of careful changes of cam rings and points plates got me to the stage where the bike was useable and I took it up the western isles of Scotland with the late Bob Brassington and two other Ariel riders in September 1997. Roland Robinson had warned me of the tendency of the OHC fours to strip the fibre gear wheel driving the dynamo and distributor so I took a spare with me. By the time we got on the ferry to Arran, my bike was rattling badly. By Mull it was worse. By Skye it was worse still and there the fibre gear stripped. So I fitted the (brand new) replacement. It lasted less than a day so I had to be recovered home. The rattle proved to be the coupling gears loose on the crankshafts and I am sure that it was the pulsation effect of this fault which caused the fibre gears to be destroyed.
A re-think was called for, once I had rebuilt the engine. The upshot was a conversion to Newtronic electronic ignition which solved the misfiring problem completely and led to much better flexibility of the engine. Basically, it took it back to the sort of flexible performance which the reporters raved about when the OHC four came out. Although basically a Newtronic system, I used a disc to "chop" the light beam of the electronic unit which was made specially for the purpose by the then Square Four Spares Organiser, David Jones. The system was so successful that it was available through Draganfly for a time - until Newtronic went bust.
Probably about 1998, Len Ore organised the first of the Forest of Dean Crazy Horse rallies in early January. I booked in and took the 4G and sidecar. En route, the bike started to misfire at anything above very moderate revs. I decided against trying to solve the problem at the rally and limped, increasingly slowly, home where the 4G and sidecar were slung in the shed in disgrace. Inevitably, to cure the misfire, I decided to fit one of the Newtronic electronic ignition systems. Whilst doing so, I found that the cause of the misfire was that the copper wire I had fitted between points end and slip ring on the armature had fractured, presumably due to fatigue over time. As the armature rotated, the broken ends of the wire were flung outward to short out on the inside of the magneto casing - a perfect governor mechanism (if I had wanted one.)
Converting the 4G to electronic ignition did not give any significant improvement in flexibility or smoothness as it had done with the OHC four. I put this down to the lower revving nature of the 4G. But the misfiring had gone for good.
When the OHC four stripped another fibre gear for no apparent reason, I realised that its magneto used a narrower fibre gear than the 4G one had. In fact, the 4G fibre gear is a third wider again that the OHC Four one was. The difference in width is accommodated within a different end plate / distributor base on the magneto. The end plate for the wider gear is fundamentally flat on the outside whereas the end plate for the narrower gear has a ridge up the centre. The dynamo drive gears vary in width correspondingly, but otherwise, the parts are interchangeable. I am certain that the narrower gears were used in 1936 on the OHC models, in 1937 and possibly 1938 on the 4G models and in 1939 on the 600cc pushrod models. Going for the wider gear makes all the sense in the world given that failure of the gear leads to loss of the distributor and therefore, almost certainly, a call for the recovery people. If you PM me a photo of your end plate I can tell you whether it is for narrow or wide gears. I use a wide gear system on my present 1936 4G, for obvious reasons.
So with the misfiring sorted, was that the end of the problems with the coil ignition system? Sadly, no. The 4G, especially with sidecar, developed a habit of the charging system failing. The dynamo was the problem and eventually I found that the solder between the armature windings and the commutator was melting and being flung off. I tried high temperature solder, a brand new old stock armature and even rivetting the wires in place but the lack of charge, or reduction in charge, happened whenever I used the headlight for any length of time, with or without sidecar. Fundamentally, the electronic (or coil) ignition was demanding more from the dynamo (even at 12 volts) than it could supply without overheating and melting then flinging the armature solder. I never did solve that problem because I sold the 4G to buy my first Healey. Perhaps you could reduce the load on the dynamo sufficiently by use of modern LED bulbs, particularly in the tail, speedo side and panel lights. From what I have heard, if you want to see where you are going, LED headlamp bulbs are not the answer. For the time being though, my 1936 4G works well on a Tony Cooper reconditioned magneto, wide gears, 12 volt electrics with electronic regulator and conventional bulbs.
Feel free to ring me on the number in Cheval to discuss if you want to. We don't live too far away from each other if you want to come and look at the 4G.
P.S. I had planned to take the 4G to Germany and Holland in 2018 but a fractured pannier bracket led to a return home for the KH which did the trip with the furthest traveled award as a result. I was concerned about the possibilities of stripping the fibre gears en route and the difficulties of replacing the fibre gear by the roadside so I had a discussion with Tony Cooper. The only way to remove the fibre gear is to press it off with the magneto dismantled. But there are 3 holes in the steel base of the fibre gear which could be threaded (say 4BA) which would enable removal and replacement of the gear at the roadside (provided you carry 3 No 4BA allen bolts or similar). I will tap the steel base of my fibre gear accordingly next time I take it off but I did have the tap and allen bolts with me when I set off for Germany, along with a couple of spare fibre drive gears.
P.P.S I do have in stock one NOS Newtronic conversion system, just in case.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.