A D&RG drawing (card 228 file L-136) gives a Date of 11-05-03 for couplers to be used on class 125 locomotives. Note says "conforms to MCB contour"
There is no reference to type, but since type D couplers weren't adopted until 1916, I would guess an earlier designation.
I could find no refrence to any lettter designations prior to 1916 in the data I have here. I did find a reference dated 5-5-03 on a D&RG locomotive pilot drawing, and a reference to cutting levers for tower couplers dated 8-25-04.