Hi Chris,
There are 2 components to the prewar clutch release. As you say - the crankcase's main bearing boss has a single steel pin. The static part of the clutch release fits over the boss and is located by the pin - it is basically a steel ring with 3 radial pins equi-spaced around it - these pins carry hardened steel rollers. The back of the ring has (usually) 3 slots to fit over the locating pin on the crankcase. The second part of the clutch release has 3 ramps (to match the 3 pins and rollers) and incorporates the operating arm (to take the clutch cable). Providing the static ring with the option of 3 positions gives a small amount of adjustment of the free movement - and the correct position has to be established by trial and error.
Regards,
Eddie.