Hi, thanks everyone. It is the front driven dynamo. I checked in the Black Ariel book, and they were indeed 4 volts. So I guess they became rare as bulbs became unavailable. I have an electrical wizard nearby who can build almost anything inside a case, so should be able to convert to 6V if I had the remains of a 4V one. The dynamo drive sprocket and cover etc are there, and the despatch book from Ariel confirms that it was exported to Norway in March 1930, and had lights fitted as extra. The original headlight case is also there, but needs some repair and strenghtening, as it seems to be made from tin foil, it is so thin. I also have three sets of front engine plates, so can experiment a little with widening the holes if I had a dynamo that just might fit. Will check whether I can get the measurements for a BSA C10 one.
1929 Ariel F,1930 LF, 1934 VA31,1951 SQ4, 1957 Huntmaster, 1942 BSA WM20, 1927 Husqvarna mod 180,1969 Honda CD125, 1971 CB450, 1971 CB750