ok, I correct myslef. NO shunt resistor. I just opend my part up. IT IS A THERMO SWITCH with a bi-metal spring sheet which opens up if heating up due to current flowing through it. Limit point seams to be 40 Amp = switch to open. So a kind of fuse with self repair capabilities. It is marked as 12V/40A in my case (4907 12/ 1972).
I added in my fully overhauled wiring a fuse for 80 amp in the main feeder line going to all other fuses and non fused part of the car, exc. starter cable. Well offering only a ONE shot to open
Matthias