Originally Posted by OurWorkbench

I'm not sure what you mean by "a manual for all these parts" as the manual found at https://ismacs.net/singer_sewing_mac...al-rotated.pdf will give you threading instructions. As for the stitch length regulator my best guess is the knob turns to lengthen and shorten the stitch length and the lever is for reversing the direction of the stitches which would be locked to the length set by the knob. Unfortunately the OP that has this machine has not been on QB for over five years.

There is a Morse and an Admiral that have this stitch length regulator, but the upper tension is in a different position than the Mercury....

Originally Posted by bkay

I'm guessing your question about "parts" is about the what the extra feet do. The feet do the same thing on all that era of sewing machines. Download the manual for a Singer 15-91 and it will tell you what the parts are for. They usually include a foot for sewing a zipper, doing a rolled hem, etc.
bkay

edit: The 15-91 manual is just a more detailed manual. There are several differences in your machine and the 15-91, but the feet still do the same thing.