KEL-TEC SUB-2000 collar types have changed from metal to plastic. All Gen 1's and some Gen 2's have the metal collar.
Due to shortages in metal, post-2021 Gen 2's have a plastic stock receiver collar.
DO NOT USE AN ARMORER'S WRENCH OR ANY WRENCH ON THE PLASTIC COLLARS!!!!
Plastic collars can be removed just by twisting them with your hand.