Which ones are you missing?
I'd like to get every visually different version, which to me includes long key vs. short key, different color backs, and a few other things. It doesn't include what I'd call typeset differences like various reverse overprints (18-63, 18-63-1, 18-63-T-1, etc.) or "1" or "a" on the bill. It is totally arbitrary, but that's my prerogative. In terms of what I'd like to eventually have in my collection, I'm missing the following:
First issue: perforated versions of the four denominations
Second issue: 50 cent note (been looking for 8 years)
Third issue: Justice green back note and red back versions of the other denominations
Fourth issue: Lincoln 50 cent note (I'm just cheap and can't pull the trigger)
I'd also like to replace a couple notes. For instance, I have every Fr in the 5th issue, but my Fr1308 (long key version) "needs" to be upgraded.