My guess is that it's a 6-volt light that someone is using on 12-volts. Used to see that a lot years ago when people wanted to keep their original equipment. Nowadays it's just easier to buy a new light.
As oldtimers like Dan and I know, 1956 was the year that almost all American cars made the switch from 6 to 12-volt systems. There was a big clamor to either buy new 12-volt sirens or use existing 6-volt sirens on emergency vehicles. Some people who were just plain cheap opted to use their 6-volt siren without making any changes; but ended up having to replace them anyway if they were overused, resulting in burn-outs.
Much to my surprise on a trip into Lubbock in the early '60s I stopped by a shop that not only sold fire equipment, but at that time they had a small shop that was devoted to the sole purpose of converting 6-volt sirens to 12. It was easy for the smaller sirens such as the Federal EGs, etc., that used the small black motors. They were relatively inexpensive and easy to replace. But where they made their money was on the larger sirens that used the "starter" motors: Sterlings, large Sirenos, Federal Class 60 and up and B&Ms. Those required rebuilding the motors or replacing them. In those days the rebuilds weren't all that expensive, so that's the way they went.
I was quite surprised to see just how "cheap" the City of Midland could be. From 1952 until 1955 the police dept. used Federal WL sirens on the left fender of the patrol cars, along with a set of red "Adam-12"-type lights on the front roof corners. But in 1956 they ended up with Pontiac patrol cars which would be the first vehicles with red Federal 17s on top. But rather than replace the 6-volt WLs with 12-volt models, they removed the light sections and mounted the sirens underhood. Most cops didn't know the difference; so it didn't take long for those little sirens to be burned up. As each siren burned out it was replaced at that point with either a Federal 76B doubletone siren or a Federal 28.
One interesting conversion I observed c.1963 was when the predominantly black funeral home in Odessa first opened, their ambulance was a 1958 Chevy wagon. The wagon was given to them by Chapel of the Roses Funeral Home, and included an old black 6-volt Model 28 siren; two red 12-volt Carpenter lights and a 6-volt 17 that had been on Chapel's '52 Chevy sedan-delivery ambulance, long-since replaced. There were no modifications to the 28, but the 17 was modified simply by putting 12-volt bulbs in place of the 6 and simple resistors like those shown above. They seemed to work well. The beacon spun a bit faster, but I never saw any problem with it during the years that car was in service.
