I should add that the large circular grommet in the picture with the single wire is where the ammeter to coil wire goes. In an original truck the wire would have been in a metal conduit, so called "armored" as an anti-theft device. I believe GM called that Electro-Lok. There was a metal cap on the coil that covered the connection to the coil.
Kent

1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix