[TR] TR4 distributor rotor size?

[J.C. Hassall]

I have a pair of 40795 distributors for my TR4, each with different 
rotors.  One measures 1/2" across the cord of the arm which carries the 
spark (the swipe arc) and the side opposite the arm is straight 
(parallel to the distributor shaft).  The other measures 7/8" across the 
arm and has an overhang (analogous to a counterweight) on the opposite 
side.  Which one is right for that distributor?

TIA

jim