So was it some sort of hall effect sensor with a sensitive microvolt meter? I can't imagine it being very or even nearly accurate.
I believe that this type DC Ammeter just used the large magnetic field surrounding the cable with a LARGE current flowing in it to disturb the magnetic field from a fixed magnet in the meter, IIRC.
So a direct reading of the field, and calibrated in tens of amps, or so.
Near as I recall (I haven't seen it in decades), it was pretty much a bog-standard moving-iron type meter movement, but instead of having a coil and passing current through the insides of the meter, the wire was held directly against the back of the meter movement.
It was certainly more sensitive than merely "tens of amps" - it could quite easily indicate the modest current of the interior light coming on and off, so in the order of an amp or so.
I think it must have been a logarithmic meter, cranking amps would take it out to pretty much full scale one way or the other, and generator (or alternator) charging current of 40A would show about 1/2 scale.
I've had a quick search of google images and haven't found a picture of it or anything quite like it. I best ask Dad next time I see him if he still has it, and take some pictures for historical reference!
OK, this isn't the same but it's very similar.