The original M400A1 FCO had a "sealed" optical assembly. Namely, the reflector, gasket, and flat glass lens were screwed together onto the door. The socket was mounted onto an assembly that had a two prong plug that fit into a receptacle when the door closed. To relamp, you had to open the door and turn the socket assembly counterclockwise and pull it out. This would make group relamping a snap if you had a couple of extra socket assemblies on the truck so someone could prep an assembly with a new lamp while the bucketman changed the lamps in the luminaire.

Looks like an early 80s model? My NOS 1981 M-250R1 with the Chicago "C" stamped on it has the same gasket and red rubbery socket gasket. See here.

This version came out in 1981 and lasted until the rollout of the A2 in 1986.

Ahh. Do you know when they started using the black rubber gasket with the painted white stripe and the red socket gasket?