Will depend also on whether the receiver is cut for the pivoting RHS "control lip", or for the later fixed style. Earlier type cannot be fitted to later receivers, but the later fixed lip mags can be used on the earlier receivers if they (can be made to) feed OK, which most do. No doubt you realise that.

Also, the C spring mags have fed better for me c.f. the W spring mags.The C springs being a bit harder to make.

If you were really hell bent on doing it, I'd get a receiver or two and machine on a pantograph exactly 1:1 the feed rails as per your mates 375, assuming it is original that is so you are hopefully copying a good average receiver. Done right, at least you have arrived at BSA's feed rail solution and not an unknown. Then survey a range of receivers (303 will do) to determine the catch height variations as best you can. Have the max, min and average on hand for testing. Ditto the catch lug heights on mags so you know what you are working with there. Then it would be easier to do them in batches to a known standard, which hopefully would become well proved in time. Not everyone has a metalworking pantograph that will cut 1:1 just kicking around, but I mention it because it is one way of doing it. Works pretty well for M98 conversions also if doing multiples on the same bottom metal and follower, and a bunch of other goodies too.

If you copy your mates mag, assuming it is original, hold the required tolerances and test as above I reckon you can't go far wrong - except the abilities of others to make them feed.

With the single stackers - if they ease the feeding issues some folk will plump for them no doubt, especially if their actions have rails that have been bastardised.