It's likely a Model 50 or 55 Riesing, made by Harrington and Richardson (our old friend H&R), a .45 ACP FA carbine that mostly saw service guarding railroad trestles in the CONUS. The Marines didn't like them much, as they had a propensity to rust quickly in the Pacific theater. Post-war, some were distributed to police departments and other local LE agencies. IIRC, Numrich/E-Gunparts had some semiauto models for sale as late as the 1980's and they also have parts available.
The uniform is Pacific Theater Marine camo.
Man, what I'd give for one of those original long Garand bayonets with a scabbard...
Regards,
Rabbit.