I would guess it right in there somewhere about what you estimated. I tried really hard to look at the frame. If the frame is steel, I would guess it between 39 and 42. If the frame is cast iron, then 1943 to a couple years later. I am guessing the tractors serial tag is long lost. There's a lot of information out there on the letter series john deeres, so don't give up. You will eventually find what you want to know.