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Not sure. Seems to be held in place by mag catch.
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It has two ramped barbs on either side at the top that snap over ledges in the lower when shove in far enough. You have to bow the sides of the insert in about 2mm and remove the mag catch before it can be removed. Since the exposed bottom lip of the insert is reinforced, you can only remove it with the gun taken apart, and with deliberate action. Very happy-meal toy-ish, but it's a proven assembly method for plastics in a zillion different products & appliances.
The only thing to worry about (which hasn't been an issue so far) is the liner bowing inward due to molding stresses, making the magwell slightly tighter. Again, hasn't been an issue so far, though you see like a paper-thin gap between the parts from that mold spring-back.
I'll second the guy who said these things are likely a better x39 magazine than anything currently on the market. So long as they don't abandon the design after a couple of years like the G36 magazine and offer it for a decent price like the Scorpion mags, I think users will be happier with them than they ever were AK mags. They won't ever dominate, obviously, but I can see savvy shooters preferring them over poly AK mags or especially x39 AR-pattern mags.