I just ordered one from Marstar this afternoon for my No 4 MkI. I used to have the aftermarket ones - I don't want to mention the mfr here, but they are squared-off and remind me of old WWII BAR mags. They don't work well, and can only hold 9, if that makes a difference. I've found an original, good-quality mag works best. They're pricey, but everywhere I've looked online, the prices are up.
And damn you Marstar for selling these mags. I perused the rest of your site and it looks like it's going to cost me a ton of $$$ now for other stuff I want, lol.
Cannon