Aside from the number of teeth, the heavy-duty flywheels were drilled for a 9-bolt clutch up through 1953. 1954 and '55 1st had 139 teeth and a 6-bolt clutch. Thereafter, all had 168 teeth and 6-bolt clutches. 9-bolt clutches are NLA and you must have yours rebuilt, as with the disc. I went through this ad nauseum last summer. FWIW, bell-housing part numbers are unique for 54 and early 55 as they changed the motor mounts in 54. Why 55 2nd and later bell housings carry a different part number, I do not know. Perhaps I read the parts book wrong.
