At present, there are no clear cut references or guidelines that deal specifically with uncontracted braille.
You'll just have to use some common sense and make a transcriber's discretionary decision.
For example: the letter indicator would have to be used in "3d" in order to distinguish it from "34".
The only other situation I can think of when a letter indicator might be needed would be with lower case Roman Numerals.
EBAE Rule VII sect. 30
Roman Numerals: ... [u] Uncapitalized Roman numerals of one or more letters should be preceded by the letter sign.[/u]
In uncontracted braille there wouldn't be any confusion if Roman Numerals were not preceded with a letter sign, but the rule above does not give us any reasons, or only whens ..., or exceptions ....
It just says to do it.
I would welcome any other comments or thoughts on this subject.