The lettering as far as I understand was not part of the model name per se. It was company code to designate a mid-run change/revision to that model. So if you needed it serviced the shop would know what part it had or didn't have or what was changed on it so it wouldn't be fixed with wrong parts. That's just what I read from members on here. I assume they probably have service bulletins preserved that tell of model changes.
The model denominations within the same series range, yes I believe you are on the right track. The higher the increment the more features and higher price point that model had.
