The "Pro Mag" Lee Enfield mags got a bad rap that really isn't deserved.
They based those magazines on the mags from the Indian No2A rifles chambered for 7.62 Nato
I just checked out a Pro Mag, made for the 303 British this morning.
I have a couple of 2A magazines on hand that were offered for sale by Numrich a few years ago.
I compared them side by side and other than the retaining/feed lips they are almost identical.
The noticeable difference is the ''slanted'' bottom of the mag for the 303Brit, to accommodate the rims
The 2A mags will work fine in No4 rifles that have been converted to 7.62 Nato, but the mag well/trigger guard need to be slightly altered to make them fit and feed properly.
It's not a big job, but it's tedious and if you have a collectible rifle, it's not something I would suggest you do.
The Pro Mag units are quite robust and serviceable and if you have a cut down, sporterized rifle, are at all handy with tools, it's pretty easy to fit them to your personal rifle and they are about the same price as surplus magazines.