Thanks for the input!My first tractor was a John Deere, "B" I think. I still have it. It has pedals and a chain drive so this is a matter dear to my heart. I pulled up lots of pictures and of the three tractors you mentioned, the only one I saw with an enclosed radiator that had the breather intake and the exaust on opposite sides of the center line was the "G". Hopefull a real John Deere man will step in and say for sure.
Model G.. grew up on one.. Bigger than the model A, and that was bigger than the model B
TractorData.com John Deere G tractor information
Stuckmotor.. The model B didn't have a chain drive, maybe you are thinking of a much older one.
Model D had a chain drive I believe.
But you pulled out the correct model G out of the hat, as the only one with the stacks across from each other.