I posted this there and they’re completely baffledDefinitely a custom build. Looks like an M1A back end with the barrel/gas system from an M1. Probably a custom op rod & guide as well. Looks like a custom gas cylinder lock too. maybe some kind of compensator? Stripper clip guide is also removed too.
There’s a couple folks over on the M14forum that have done this. They reported great results too.
But that’s pretty much just a an M1 with a detachable magazine, it’s not the weird Frankenstein rifle this one isI’m surprised they didn’t know. Here’s a post on such a hybrid
M21 Look alike and HYBRID M1A and M1 GARAND
www.m14forum.com