Rob,
J11C comes up as a 14mm, 3/8" reach, 13/16" hex head, copper core, which would be the bigger plug.
They appear to be the same reach and thread diameter as the Champion 871's I have in the '46 2 ton, and had in the '50 1/2 ton.. the only difference between the J11C and the 871's is that the 871's are resistor type plugs to help reduce radio frequency interference. (static in the radio).
