I'm fairly certain the main difference is that the Generation II mags will have a cylindrical chunk of rubber at the floor plate on the rear of the magazine spine. IIRC, and I should because I have 10 at home, the rubber is held captive on the floor plate. Anyway, it is basically wedged between the floor plate & the magazine body so as to keep the floor plate from slipping backwards, or coming unhinged, and dumping the spring, follower and any ammo that is in the magazine. Before HK added the rubber piece, there was nothing keeping the floor plate from slipping backwards if dropped.
The date code consists of two letters stamped on the magazine where the magazine catch engages the magazine.
I hope my recollection was correct and or nearly accurate, I don't use the HKs as they're heavier & I have dozens of standard USGI mags which have yet to fail me. But I'm in agreement, the HK mags are super nice.
Sly