>>63980795
It's not wafer thin, the "tongue" is a pretty thick piece of steel which attaches to the front of the receiver, underneath the trunnion, with the piece resting up against the barrel.

Here's a page showing disassembly so you can see the grip removed from one:
http://www.nfatoys.com/tsmg/web/m1assy.htm

Here's a surplus 1928A1 piece listed for sale, so you can see its shape and dimensions:
https://www.worthpoint.com/worthopedia/ww2-s-army-1928a1-m1-m1a1-thompson-434905123

I'm sure someone has once managed to break one of these (like not counting someone being hit with an artillery shell or crushed under a tank or something), but it can't have been fucking easy. When fully assembled, this should easily be able to support the weight of a fully grown adult GI, like if he somehow had to hang on to it for his life to not drop down a cliff or something.