Correct magazine for a 1960's / 1970's British used M16?

[fjruple]

I would imagine the twenty round magazines from Colt would have been appropriate as the magazines would have been part of the rifle sales package to the UK. It quite possible that other 20 rounders or 30 rounders "found" and "borrowed" their way into the UK system via the many joint NATO exercises that took place.

--fjruple