Steve,
I went out and snapped a couple pictures of some of 101 Jrs out in the shed and am now more puzzled than before. I was always under the impression these stamped letters were for the type of gears in the rear end but now am not so sure about that.
The one thing I came up with is that the lower the serial number the lower the letter and the newer tractors seem to have a B,C,E and so on. Do you think possibly they started with some sort of a lettering system similar to what the Case tractors had to signify the year it was made?? Here are just some I looked at:
J R A 397199
J R B 500691
J R E 504095
J R E 504200
What do you think??
Joe