Hello all, can anyone please help?

I have several Tung-Sol VFD tubes.
They are all P/N DT-1704, but some have the letter A, B, or C, on the
end of the number. (IE: DT-1704B, etc).
Does anyone have a datasheet for these tubes or can you tell me what
the letters designate?
I'm looking for any documentation that describes the difference in the
letter A, B, C used in these part numbers
Thanks, Kevin.