This mag resembles the Brit mags on the market.
MADE IN USA
Parkerized finish like the Brit, about 3/4" inch longer. Steel body, much thicker than what I recall a USA mag as for the steel. Small round hole above mag catch hole. Another small round hole in other side directly across.
What brand is this and are they any good?
What you describes sounds like another aftermarket mag that someone put a green follower into. Buy USGI only!