I'm guessing you are referring to part number 125056. I have two parts lists that have illustrated plates for it. I haven't had either of my 15-91s that far apart. Nor have I seen anyone refer to it. Looking at the parts list, I'm not sure how it gets in there and the adjusters manual doesn't seem to help much. Or at least to me. My suggestion would be to print at least the page that has part 125056 on it. I actually have two parts lists that show it. One has it doubled over the other is single. Both appear to be full size plates, which you could measure from the print out. I know the full size illustration plates come in handy. I used it a lot when working on my sister's 66-1.
I wonder if candle wick would work. I know they have different sizes of wicks.The wick for the motor is 3/16" diameter (according to
https://shop.sew-classic.com/Motor-B...nch-WICK-3.htm and appears different than what is in parts list.
Good luck.
Janey - Neat people never make the exciting discoveries I do.