Two major changes occurred in 6 volt General Motors sealed beam bulbs (are actually not sealed beams) since they were first installed on vehicle assembly lines in 1940. The perfectionist restoring his truck to exact original specifications must have the correct headlights for his year. In very competitive judging, it’s these details that can make a difference. No doubt, replacement bulbs from a GM dealership with slow sales could sometimes be placed on newer vehicles a few years after the units were discontinued, however this article is based on bulbs you would have bought new from the factory during that particular year.
1940 through about 1955 – These headlights had a double filament small bulb built inside. The large glass reflector in the back was sealed from the elements. It stayed bright even after the inside bulb burned out. It was not like earlier open reflectors that could tarnish with age due to the silver plating. The assembly comes with a metal black back attached for support. If the outer glass gets a rock hole, the light continues to work well. The filament is still encased in the smaller argon gas filled glass assembly (see photo).
1940 through about 1946 (above)
About 1955 and Newer (below)
The first truly sealed beam bulb, as we know it, was introduced about 1955. Between the reflector and the outer glass covering is the open unprotected filament (no small internal bulb). The total interior is filled with argon gas to protect the filament from air which causes instant burn out. When a rock places a small hole in the glass there is instant burn-out
One of the big visible differences in this first series sealed beam bulb and today is there are the three aiming bumps on the outside in about 1955. The bumps were necessary when using the new dealer aiming equipment.
1955 and Newer (above)
Beginning in 1955 the 6 and 12 volt sealed beams have the three glass aiming bumps molded in the edges of the lens. The bumps were needed by new light aiming equipment provided to most all dealerships. All of these GM bulbs with aiming bumps have the letters T-3 molded in the center of the glass lens. Most sold by the GM dealerships will also have the word Guide at the top of the lens.
Note: These modern bumps will interfere with properly attaching the chrome factory bezel on a 1940 headlight bucket as well if a 1937-1939 bulb light that has been converted to sealed beams. The bezels were not designed for the bulb still 15 years in the future. The 1940 GM vehicle owners will have a long hunt to find sealed beam bulbs without the three bumps.