It's an LB Comp, with the cut-out for the 12g.
Nightmares only lived up to the name when you had to fix 'em.
But there's an old-school trick for putting them back together.
You need what Machinists call Brass Shim-Stock.
You wrap the valve assembly with 0.015 Brass, lube it up well and slide the wrapped valve through the threads of the body, then pull the brass "sleeve" out.