QUOTE(vader897 @ Oct 25 2006, 23:19)

This has allready been suggested but I am offering a slightly alternate version.
You could use two tags... one for the number and one for the side... this would mean there is no redundant data being kept. (not that it will make much difference.
ie %side%
%track_number%
in %side% I would place only the data "a" or "b"
I would then put these two pieces of data back together in whatever application you are using to display ur a1, a2, b2 etc...
This solution will not work as I read it. The sequence you have given would not occur (A2 and B2, the second track can only be on side A or side B, not both). The idea is interesting, but it would introduce an entirely new numbering scheme that would probably not be readily apparent to an uninformed viewer.
I'm afraid my initial response might not be clear, so allow me to try to rephrase it.
First, follow Yotsuya's explanation of just translating the A1, B3 etc to their tracknumbers, as they would appear if released on a CD.
Then second, make a new ID3 field for your somewhat redundant
vinyl track information.
Now, in practice, since you have the normal
tracknumber field, you can use that for ordering and sorting. You've met the practical goal. However, for display purposes, you can instruct foobar to use the
vinyl track field for display information if it is present, instead of the
tracknumber field. You've met the aesthetic goal thusly.
I think this solution is the most flexible and meets all your desires. The only downside is slight redundancy (i.e. you have track number information twice, in 2 different formats) and space (a few extra bytes).