I built 1 off of a virgin FMP receiver and one off of a virgin SW3 receiver. Both were the old style feed mechs.
Both were Cali guns. You could build from a 91 or a PTR .
(1) was stamped sheet metal (1) was milled .
I had them built by Ralph at RDTS .
You can try Michaels Machines ? He makes the E versions
There is a guy that is in Calif. Riverside area that is building them from all virgin stuff. NEW barrels and everything.
Building E versions of both the .223 and .308 as well as a convertible version. so you will be able to switch calibers.
Not sure if he will do partial build stuff.
I'd kind of like one of the converto E style myself.
You do know your unicorns welcome to Dreamland.