could be a J (61ci) or a JD(74ci) or a JA(police) orJB (high compression) orJE(racing) or even a JS or JDS( the s is for sidecar)
I had a 1927 JD back in the mid 1960s.It was interesting to ride,needing to stop every 40 to 50 miles to give the valves and rockers a shot of oil.Also the oil system was total loss,it had a pump but only put a small amount into the engin,when you needed it to pull more power you had a hand pump on the tank to add more oil.