Text copied from TM-E 30-451 , March 1945 :
This consists of a cap, with twin leads of copper or iron, and a wire bridge.The whole fits into a standard bakelite cap holder. The copper leads have a resistance of 2 Ohms; iron, 3 Ohms.
Brass type: Length 55mm, Diameter body 14mm, Diameter collar 16mm
Bakelite type: Length 53mm, Diameter body 16mm, Diameter collar 18mm