Replicating the M21 "Fire Selector"?

[LzChase]

I'm.... actually not sure what that thing is or what it's called, it's not a fire selector for obvious reasons, it seems to either just cover up the place where it used be for looks, or have something to do with semi-locking itself. But how may I replicate this part for an airsoft build?

[Lozart]

M21 rifles were usually converted from National Match rifles which have no fire selector and the cutout in the stock is filled in. The ones like you've shown look like the standard fire selector with the outer "handle" cut off flush and filled in. I'd suggest that could be your best route. Get a replacement selector and cut it off flush.  Bit of paint, job done.