Originally Posted by Junk Man
It looks like a copper strip with a slider for one of the legs. Is it really that simple? What would the gauge and dimensions of that strip be? Any ideas?
It would not be copper because the resistance is too low. It would be a strip of resistive metal, thick enough to handle the total load current and long enough to generate millivolts between the 2 sense points. The shunt also needs to keep that resistance stable through a reasonable temperature range for meter accuracy.
This link may be a bit to technical, but it has the basic info.
If you are trying to make an exact replica, you are better off finding one in a bone yard. If it were me, I would find a commercial shunt that matches the meters needs.