A.R.
Part numbers and some casting numbers are here. (this info might have been posted above)
The 6v/12v 139/168 teeth flywheels are poorly/confusingly delineated in that Master Parts Catalog (for 1955 1st and 2nd, 1/2 ton vs larger trucks).
Assume that 1955-2nd Chevrolet trucks are 12v with 168 teeth.
Here are the part numbers (no casting nos):
8615574 - 168 teeth
8615138 - 139 teeth
(corrected number- thanks)