Seems a lot of Promag stuff is like that. Their Lee-Enfield mags are not the right shape either. They look like they put a .303 follower into a Mk 2A/2A1 mag(similar to FAL and M14 mags). Different rifle, altogether, that was never made in .303.
If you find a mag for a Mk III they can be made to fit a No. 4 by grinding off the bottom locking lug.
Bob's Gun Shop lists Enfield mags at $35 each, but doesn't say which rifle. Wouldn't hurt to e-mail him and ask. Says a response won't be quick though. Busy place. email@example.com
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