This cover of Beat Happening's Indian Summer turned up on a 7" single given away with issue #7 of Chemical Imbalance and was recorded while Galaxie 500 were still a going concern. I don't think it gets a mention in Black Postcards (although I don't have a searchable version as yet - a Kindle issue would be nice sometime - particularly as my hardback is falling apart - a good fan would have taken more care of his signed copy!).
Damon mentioned it in the Ptolemaic Terrascope interview but only in passing...
Those other songs you mention ‑‑ ‘Indian Summer’ and ‘Smile’ ‑‑ are things Dean did on his own, before the band broke up. (Like we with our Pierre Etoile project.) ‘Indian Summer’ is by Beat Happening, I can't remember what he did that for, and ‘Smile’ was something Dean did for a Shimmy comp while we were working on the last album
Of course Indian Summer was to become a staple of Luna's set throughout their life - they played it as their first London show in 1992 and was the last song they played in London in 2005 (and the second last song they ever played).
Dean Wareham - Indian Summer (mp3)[audio http://media.fullofwishes.co.uk/05-dean_wareham/audio/03_Dean%20Wareham_Indian%20Summer.mp3]