Milton
The S/N is not on the transmission, but stamped into right side of the gear reduction housing, the cast part between the transmission case and the front frame section. If you cannot find the S/N there, I would say that the reduction housing has been replaced with a new one (rather than a used one) at some point in the past, rather than the tractor excaping the factory without a number. However, I have a "homemade" cub cadet that was being built,by a local welder. He liked the 70/100 Cub Cadets, but because he only had one arm, they were not user friendly for him so he decided to build his own version. Due to his health issues it was never finished, and sat in his garage from the late 60's until I got it a coupla of years ago, after he passed away. Everything on the tractor was new at the time it was installed, including the wheels, tires, metal,engine etc. The transmission is for a 70/100 but does not have a S/N on it, has never been painted, except for the casting sealer IH used, and has never had any oil or fluid of any kind in it. I will not speculate as to how the builder may have come by a new transmission, but perhaps your "O" came by a new gear reduction housing in a similar way.