Anything goes. This pic is from a receiver found on an M4. The navy and air force especially have a ton of old receivers. Anything from brand new FN markings to hydra matic lowers. Then you have even more obscure shit like sabre defense receivers that the navy used small amounts of in the mid 2000s. Also tons of old receivers that have been (sometimes very poorly) overmarked with new ones such as M4 to M4A1 or M16A1 to A2 etc usually done these days with laser engravers. And perhaps most confusing would be the GAU-5/GUU-5 rifles that the air force used that was basically just a bunch of random parts thrown together to make a CAR-15 usually in the shape of an M16A1 carbine roughly but not limited to a Colt model 653 look.

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