Sorry I'm late to this party. Nobody ever answered your question on a repair manual.
Your in luck! Compliments of the U.S. government!
I can't give you the link, but, tpub.com, somewhere in Aviation Maintenance,
Preventative Maintenance, Chapter 4 sewing machines, is a FREE, pretty complete guide for 111W155 and 31-15.
Really bad spacing and font but you can read and great info.