David....well in the nicest way I respectfully disagree
that tightening the acorn nuts first locks the rocker arm solid. Or should I say that in my case I fully tightened the acorn nuts without the 'adjustment screw' being in place, and I still had 6 thou clearance on both shafts. This is the method described in the owners manual which states the cap (your adjustment screw) is only there to seal the end of the shaft.
It appears to me that the assembly is designed so that the shoulder on the end of the centre section of the spindle (the machined part the rocker arm rotates on) is pulled up tight against the distance washer when you fully tighten the acorn nut. This clamps the washer between the shaft and the casting boss. The manufacturing tolerances of the shaft, rocker arm and distance washer provide the necessary clearance and the shaft is then rigidly located laterally.
If you pull the spindle across / locate it by screwing in the end cap first you create a gap between the spindle and the distance washer but the shaft and washer are no longer clamped in position against the casting boss. Whilst it is true that this will provide clearance it means that the spindle is now secured and located laterally by the end cap which itself clamps against a less than solid fibre washer. I would have thought in engineering terms this is less than desirable compared with the very rigid assembly when you fully tighten the acorn nuts first and clamp the spindle against the boss.
There will of course be cases where the tolerances result in the rocker arm binding (although with the later setup there appears to be so much clearance that it is hard to see that setup causing the rocker arm to bind.) With the earlier setup, as Nev says, you can then take a thou or so off the ends of the rocker arm to get the required clearance.
The introduction of the altered spindle and spring suggests that there were problems with the earlier setup..although it had been deemed OK for the previous 10 years. Manufacturers do not normally alter components without a reason. Was the tooling for machining the faces of the bosses in the boxes getting worn? who knows. Whatever the issue a simple fix was to shorten spindle shoulder and add the spring which would allow for greater tolerances in the components.