Back in the 80’s when fine point restorations were peaking and restored original starter solenoids brought a premium price, I made all the tooling to rebuild them. The internal wire size (gauge) in the coils was the same on both 6 and 12 volt. The number of windings differed only by manufacturers. As stated, you would have to do a bench test to see whether it was ground activated. That was what created problems for Ford owners because most all other makes used the same appearing solenoids. Some of them were stamped on the mounting bracket 12v, but I don’t believe it made any difference. I always was interested in cores which had the push button on bottom, screw on cap and the letters RBM stamped on the bracket as they were the original manufacturer of solenoids when Ford went to the dash mounted starter button in 1937. In 1939, I believe Ford shortened the height of the case yet continued with some of the early features before going away from the button altogether. I still have many cases but cadmium plating became so difficult to get done, I just filed the old tooling in the obsolete corner. It was fun back then.
