This guest appearance by Robyn came to light with the publication of Dean Wareham's memoir 'Black Postcards' - see image and transcription below.
Dave Rick jammed with us, and Robyn Hitchcock was in attendance for some reason. He jumped onstage and sang "Rain" with us, the only song we were sure we all knew. The next day we listened to the recording of Galaxie 500 with Robyn Hitchcock. We sounded dreadful, but his voice was great. This little collaboration was mentioned on MTV-we were almost famous.The exact date is not mentioned, but Galaxie 500 played The Rat on this date and 'Rain' is listed in the setlist
on fan site A Head Full Of Wishes