In 1948, GE produced the first-ever mercury-vapor fixture, the Form 109. It would be followed by the Form 400 in 1955. These remote-ballasted fixtures are most commonly known as "bug lights" or "clamshells," because of their rounded shape.
The Form 400 spawned a larger lookalike, the Form 402. While the Form 400 was capable of carrying light sizes up to 400 watts, the Form 402 could use 700-1000 watt lights!
GE's remote-ballasted fixtures are known to look like blimps, or, as Doc Johnson would say, barbecue grills, as in the Weber favorite.
In 1959, GE jumped into the cobrahead arena, producing the M400 (used for city boulevards and metropolitan expressways) and the smaller M250 (used for residential streets and back alleys). The early M400 and M250 are known for their protruding front ends that extend over their refractors. There was also another small fixture, called the M100, that looked like an upside-down nut scooper and lacked a refractor (it was a full-cutoff fixture).
In 1967, the M400 adopted a more-simplified look, and it lost the protruding front end. This design, known as the PowrDoor, is the streamlined look that we're all familiar with. The M250 also got the same PowrDoor treatment. Most of these original M250 PowrDoors are still in service on the side streets of Baltimore and other large cities.
A larger fixture, called the M1000, was capable of 700-1000 watt lamps. This luminaire looked like a supersized M400, and earlier versions had the fins on it. As newer M1000s were made, the fins were eliminated.
In 1980, a full-cutoff version of the M400 was introduced. Its main characteristics include a protruding front end and the lack of a refractor (though some CrimeFighter versions may exist in Chicago).
During the mid-1980s, both the M400 and M250 underwent some minor facelifting. The M400 would adopt a flat-bottom design that was reminiscent of the old Westinghouse Silverliners in their final days.
In 1997, the M400 took on a totally new design, at least in the GE sense, anyway. It now shares its look with the American Electric Model 125, but with some differences. For example, the new M400 has thicker doors that give it a more muscular look than the Model 125, as well as improved mechanisms.