Strangely enough, yes, I used "first up-vote" when I meant the "first" "up-vote". I'll try to be less literal if it will be clearer to you. ;)
I don't see how your other interpretations make much sense as ways to do things, though. It sounds like you were trying to guess at the implementation and reluctant to consider that it might be easy to distinguish the first X-vote.
The current system already tracks first up-vote by having a separate field for 'post bonus given yet?'. Since then we started tracking total number of votes cast for each node, which can be used to detect both 'first' votes even without that other field.