Giz,
The taper on mags fitted to post war Douglases is the standard Lucas taper, but the boss on the mag wheel is usually the full length of the taper (as it goes through the oilseal in the front cover). The taper on modern mags may be a bit shorter, resulting in the wheel sitting against the shoulder before it bites on the taper. As the new mags probably don't rely on a similar oilseal, it may be possible to shorten the boss a little to make the wheel fit.
Regards,
Eddie.