I just got a Thermold mag from a seller on Gunbroker & it won't lock into the magwell of my Bushmaster. It's marked H12 & on the left side of the mag, the square notch isn't there. I have a H22 I use & it does...Anyone had this problem before? The dealer is going to give me a refund & I'm going to ask him to compare & if I'm right, just send another one. I like Thermold's; the one I have works perfectly! & they look cool :-)

Yes it had a slit on it. Thanks! I sent it back already. The seller is refunding my $ & I asked him to compare to other Thermold's. For the M-16 I'd only read about H21 & H22 not H12.