Quoting LEGO Lord
Ah this is pretty nice too, like your previous one. The one thing that was a bit off is that pan piece connected to the back. When attach to all those larger pieces with not much of a pay-off, it looks peculiar.
Do you mean the round piece on the side next to the 2 red lines? If so, that's my version of a fire selector. I have one on my recent MP5 as well.