The few BSA manufactured 375-2 1/2" mags I have seen were double-stack, not single-stack. When cutting them down, it would make more sense to maximise capacity and keep the original configuration. Done properly, a single stack conversion probably would be more laborious, reduce capacity, increase weight and be a solution to a non-existent problem provided the receiver and mag on donors were not half pregnant from previous conversion efforts.

Visual inspection of the feed rails on a handful of BSA actions I have seen are machined alike (but different from the .303), and not heavily worked over by BSA after that machining. Combined with appropriate original mag mods, it seems fair to say they were not hard for BSA to get to feed.

However, I'd reckon to supply replica mags for the 375 2.5" you'd need an original un-molested receiver on hand, and the mag to duplicate, so that you can test each conversion for function in an original receiver. That assumes that the mag sits the same depth in every receiver, which they don't, there is a bit of variation. Then of course, the feed rails of the end-user's action must be modified to suit, which you have no control over unless you do that yourself and test. Then, what bullets are to be made to feed?

With the "short" .303 mags, the original LSA mags were very much different from the BSA "roach belly" magazines. I assume from the below, that you make both variants?