I am going to rely on a 45 year old memory, but I am thinking that the Model 15 and
Model 20 machines used by the Associated Press in North Carolina had the same
main shaft and motor gears.
I think it was done this way so that if we had a motor failure, we'd just swap a motor
and not have to worry was it a M-15 or M-20.
Other wire services or even other states in the AP network might have had different
gears between the machines.
That would suggest the baud rate might have been slightly higher on the six level
circuits.
I am also thinking that the range on the range finder was slightly smaller on the
M-20s than on the M-15s.
On the other hand, I might have a bad memory.
Sheldon