Nice! The M-400 didn't come out til 1959 so it can't be before then. It's an early one though since it's got the orange gasket, blue nameplate, and red-varnished ballast.
I'm restoring my 1978 GE M-400 split door back to it's original style with the door with no vents and clip to hold it closed and no p.c socket just a clear hole cover. The only difference will be the ballast since the original was 277v I had sold it and put another GE ballast in. In fact I'm trying to get Utility metals to fabricate me a bracket just like GE's use to be and I might put it up.