I think it depends a bit on what the letter/number references mean. You could consider it a cross reference but I think it would get lost. My suggestion (and it is JUST a suggestion) is to place it following the exercise number and enclose it in parentheses. You would have to do a transcriber’s note regarding this change to print. Something like “References that follow exercise questions in print are enclosed in parentheses and placed after the exercise number in braille.” Whatever you decide to do with these, just be consistent 🙂
Cindi