[JSeery]

I am not familiar with what type of ammeter is in a 51. There are loop ammeters, wired in ammeters and voltage indicators. Looking at the wiring diagrams, it looks like a loop type. If it is the loop type the test in the OP are a bit baffling. Would it be possible to post a photo of the ammeter in question? (the wiring diagrams do not always match up with production).

Found some photos of F-1 gauges, and they are the loop type. Also found 1 photo that claims to be of a 51 passenger car, and it would be the wired, shunt type.