SKS or AK47

A receiver flat, is just a stamped receiver. It is flat. You have to bend it, then weld/bolt in the lower rails... It's all actually easy-peasy... Whole lot online to show you. Used to be, parties. Everyone shows up, everyone helped out. But I guess the gubment said you can't do that anymore.

I don’t know about a Remington 760 or w/e model was thrown out there. You can still get 80% lowers and pistol frames that require tooling but have no serial number.