This info comes from Kevin Dockery's book 'Weapons of the Navy Seals',
Colt had a thirty round mag available since at least 1964, it was a fully curvred designed and would not fit all AR lower receivers. In 1968 Colt signed a contract with the Army to supply 1000 new 30 round mags. By 1969 the 30 round mags were begining to become available with the Seals being amoung the first to receive them.
If you can find this book it is a wealth of info on Seal small arms, I highly recommend it.