Possibly the revision of the mag's follower (guessing here). I know Glock mags, have revision numbers, marked on the followers themselves, so you know what generation/revision the follower is in the mag.
Thanks for that original post. I'd not noticed that before on my CMC Power Mags' bodies. I bought mine all in one batch and I've got a number "11" at the top of the left side, below the "45ACP".
I suspect it's an indicator of what machine made the body, but don't actually know.
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I like the "year of manufacture". I can't remember exactly when I bought my Power Mags but it could have been 2011 which would explain mine being marked with the "11".