I just picked up another one toady, didnt plan on it but what the heck I could always use another, anyway. Its factory, not a Hi-cap, just regular factory replacement. When I got it home it is different than the two the gun came with in the box. It holds its 10 rounds and fits fine, it just has different molding around the top portion, its actually thinner towards the top. Also there is an extra piece of metal inside down the length where the tip of the rounds are. They both have the same part number, long story short--anyone have information on revisions that may have been made? or the reason they would thin it out and add the metal?

the mag you purchased is a newer style mad,

back when they made your older ones they designed them without metal inside so they would swell when loaded and wouldnt drop out of the gun till it was empty or near empty,  they called these dropp free mags!  the one you just bought whichis a new style  and revised, Glock added a metal sleeve in the mag  this is simply another design this one will not swell  there for will drop out of the gun with a fully loaded mag
most prefer the metal lined mags over the Drop free mags,  the drop free mags can be a hassle sometime s if you ever did have a jam of some sort (not gonna happen in a glock )  you would have to pull the mag out instead puch a button and eject it

hope i answered oyour question somewhat i tried
Glockster19

That what I wanted to know, thank you sir !