No, there was no Insul-brite involved. Thank you for your guess. Only cotton fabric top and bottom, with a layer of Hobbs Heirloom 80/20 cotton batting in the middle.
Hint #3: The problem wouldn’t be specifically to any one brand or type of vintage machine, and the solution would be effective for about any brand or type of machine.
Are there any other guesses before I give the whole story?
CD in Oklahoma