What is a 12v fly wheel or 6v fly wheel never heard of it
The simple/practical answer is:
A 12v flywheel is one that came on a 12v truck.
168 teeth on the flywheel is 12v.
A 6v flywheel is one that came on a 6v truck.
139 teeth on the flywheel is 6v.
Both the 6v and 12v starters have 9 teeth. The difference is in the diameter of the drive-gear on the starters.
The second post here has a good explanation/description. 
