I just received a bunch of these from a fellow that got 75 new ones on a deal for some other stuff, and advertised them on the 'other' AR forum's equipment exchange.
It was a real breath of fresh air because he didn't gouge us on the price.
$13 + $5 for shipping up to 15, and $10 shipping for anyone buying more than 15.
They all went within the first few hours
I'm in California so he also took them apart for me so they could be legally shipped in as "Parts repair kits" (we can't have any mags shipped in that are over 10 rounds).
I got them because they have the tan anti-tilt followers, and are to be used in my 50 Beowulf which will single stack 10 rounds in them, and the anti-tilt follower keeps them from standing up when loading as well as just feed better.
So ..... I need to permanently mark them with "50 Cal Beowulf, 10 ROUNDS" before I reassemble them, make a slight mod to the lips (to keep them from scratching the coveted 50B brass) and I'm GTG in Ca. ....... as long as I never put any 5.56 in them!
But yeah ....... they are made from reasonably thick alluminum, with a thick spring and (IMHO) it's no wonder why the military's buying a bunch .....