This is the GE M-1000. This is the 1000w version of GE's M-series of cobraheads. This looks like an early one. There's no bolts on top and this has the older version of the door. this may actually have a ballast inside it. The ballast can may have been for the previous light.....
No Michael, this is not the earliest one, but it is a late 60s-70s version, the earliest one was silver colored one with a FIN on top for room for capacitors! They changed the GE M-250R and GE M-400 in 1970 in the fixture designs but kept the GE M-1000 design longer...until closer to the 80s, not sure exactly what day. Note it has a thumb latch...
i thought the M1000 was discontinued in 1987?