What he said about the butt stock door, it rotates.
To field strip it, your right about the pin that holds the lower on. If I remember right, it's a stock pin that has a screw added to one end, you need to remove the screw then knock out the pin. Oh and make sure the striker is in the forward position, the fired position.
After you get the pin out the lower slides off, then pull on the charging handle to get the bolt & striker to come out of the receiver.