Rick, Thanks for the reply.
I'd considered marking mags, but haven't yet. On the night in question, I threw the offenders in a separate pocket of my shooting bag. When I got home, I could feel the tendency of the follower to tilt forward on 3 of the 4 mags in that pocket.
Two CZ-85's. One that I bought new about 5 years ago, the other I picked up used a couple of years ago. The older (used) one had a known problem with premature lock-back. I'd just had them both in the shop for a look-see. The night the problems occured, I'd shot the newer gun for 2 stages, then switched to the older one for the last 2. The problems were all with the older gun.
The problems occured near the end (2-3 rounds remaining) of the mag.
Ammo is the same for the past 4 years (my reloads.) The gunsmith complemented the ammo as consistently accurate. (He test fired a couple of mags from each gun.)
No other changes. The mag problem has been around for a while. I just decided to get serious about fixing it.