When were the first HPS cobraheads introduced? Was there one company that started making HPS before anyone else? My 400W MV M-400 seems to be an oddball. It has vent holes on the door and just a screw (no safety latch). Not until you open it up do you realize it's not an M-400A! It would definetly throw somebody off if they were looking at it from the ground.
General Electric's Powr/Door luminaires first appeared in the mid 1960s along with a new grey finish for their M-series single door luminaires. these new luminaires had a captive screw and no vent holes. The vent holes likely appeared in the early 70s and the safety latch appeared on all M400A1 luminaires in the late 1970s.