I also had never seen a USGI pre ban mags that are dated but according to people that are in the business buying and selling these 30 rd mags, most of the US military mag suppliers don't date stamp their mags except two sources which are Center Industries and Okay Industries.
Center and Okay both made unstamped as well as stamped mags.
Center Industries are externally marked with their CAGE code of 6P199 with two digit month, slash symbol and two digit year.
Okay Industries have two versions of markings, the early markings are internal only, the letter "O" then month on the left half of the body, then two digit year next to the month and at the right half the letter "K" with month plus the two digit year next to the month stamp.
The location of the internal stamps are at the bottom of the mags about half inch inside the bottom edge or floorplate, the numbers and letters are also upside down in orientation, but when you look inside the mag upside down, the markings are right side up.
The Okay internal stamps was seen around 1986 or even earlier, up to early to mid 1991.
The second Okay version of markings are both internal and external from 1991, the internal marks are the same as the older version but the external markings are the Okay CAGE code of 33710 and month/ year.
I had 30 rd Colt mags that are made by Okay Ind. and most of them had the internal markings but only a few don't, my pre ban mags I bought them from 1984 up to early 1994 before the ban.
Currently all my Colt pre ban mags I no longer have, usually I gave them away to relatives and friends in ban states that allows pre ban mags. I've got plenty of mags now, my USGI mags are made from 2005 to recent mostly D&H with Teflon finish all upgraded with Magpul gen 3 anti-tilt followers, a few steel various brands, PMAG and Lancer translucent.
The mag sellers are also saying that the pre ban mags with external date markings usually have a higher price than the mags with only internal marks.