I didn't explain my query very well, in that on my M20, the mag pick up is as on Rik's left hand photo, suggesting that my mag is a late 30's to mid 40's mag. However, my mag has slack wire advance, which I thought meant that it was more likely to be post war.
£720 is an expensive 'chance' to take....The fact is it may work OK or it may not...
However, if you already have a magdyno that doesn't work, you can have both units fully reconditioned using modern rewinds etc. etc. for quite a bit less than that...
Patrick, both the mags in the photo are machined with the pickup in the same place - on the opposite side to the engine. Nortons were odd in having the pickup close to the cylinder - maybe warm and dry but a bugger to get at.
Nortons had clockwise mags with cable entering on the left for tight-wire advance and their points assemblies are quite hard to find as they were often cannibalised in the 1950s and 1960s to provide slack-wire advance on the anti-clock mags used by the majority such as BSA.
I still hear of Gold Star owners looking for Norton type assemblies as they're all afraid of their temperamental beasts ending up on full advance by accident.
The only difference between 'early' and post-war mag bodies is the casting with that 'dip' or cutout along the top face where the dynamo sits.