... locks back manually, but not after the last round. Should I waste time trying to fix it? I need more magazines anyway. All I have are Mec-Gar 18's and a handful of Beretta's, so I'm not sure where this one oddball even came from.

I think I can see the problem when comparing it to other magazines: the slide-stop actuator is visibly lower. I suppose the only fix would be to replace the follower?

Incidentally, I have only tried it in one gun. Maybe tomorrow I'll test it in some others. Still, it fails to function in at least one pistol, so that restricts it to range use.

IF you were to get it to work, would you trust your life to it? I would maybe save the spring/baseplate and toss the body. Mags are cheap compared to a failure at the wrong time.

If it's a Factory Beretta/MDS magazine, before tossing it disassemble it, and make sure it doesn't have the magazine-spring backwards or upside down.  It's very, very common for folks to install the magazine spring backwards after cleaning, something I've done in the past and is very easy to do.

Make sure the top of the spring (highest point/coil)  is pointing FORWARD to the front of the mag body when the follower is resting on top of it.  Hope this makes sense without pictures.  Good luck.

It is of unknown manufacture and has no markings, other than "5, 10, 15." Like I said, I don't even know how I came to possess it. It is assembled correctly. I think that, since it is a 15-rounder, I will replace it with 15 new Mec-Gar magazines.

Take the spring out and stretch it out a bit.  If it begins to lock back you know that the spring needed to be stretched or was on it ways out.  Just don't keep it loaded up for when you NEED your magazines to work.  Range mag its just fine.  If you want to toss it let me know i'll retrieve it.