I perhaps should have been more clear: the previous owner heavily modified the panel. I do have two dimmers, but one is for the engine gauges (internally lighted, EI gauges if I recall), and one is for electroluminiscent strips above all the steam gauges. There is also a separate switch for those, and that one is clearly not in series with the nav lights. I don't have an additional dimmer on the panel or in the overhead. That said, I did not try adjusting the dimmers while I had the meter on the sockets.
There was no light bulb, no torpedo, nothing; just this socket connected to a ball joint, that in turn was screwed directly in to a bracket that was attached to the overhead panel. What I'd like to do is track down something that'll fit that socket, so I can attach it when I put the interior back in; alternatively, if that's not possible, I'd like to find out now so I can investigate how to replace it with something more modern that'll do the same job but be LED. Also, for the record, the wires for these come directly out of the panel and don't go through any rheostat ; one ground wire attaches the bracket directly to the airplane, and two more wires run into the center of each of the two sockets. Also for the record: when mounted, the sockets aim out the left and right sides of the plane, not forward, and I do not have a piece that attaches to them; no torpedo or anything like that was included when I purchased the airplane 3 years ago.