I've got a reference book on 1873 Winchesters at home.
I believe the letter concerns the variation of the gun. There were three different model variations, having mostly to do with the dust cover. Yours is probably a third model, as mine has a "B" also (IIRC) and it's a third model.
If the track the dust cover slides on is integral to the frame, it's a third model. The second model had a screwed on track, and I can't remember what the first model had. Of course, this applies to both rifles and carbines. The action was what varied. HTH