If you've already got some assorted screwdriver bits that will fit parts of your Singers, and a bit-holding screwdriver handle, you don't need to purchase an entire set. You can save a couple bucks by ordering just the individual bits you need from a company like Brownells. So what I've done here is photographed the various parts of a 15-91 and labeled, below, which Brownells bit will fit it. (I figured looking at photos of the screws in question is easier than figuring out the name of the screw and checking that against a list.)
You can then order the bits you need at this link to Brownell's website. They're $2.99 a pop. Happy fixing!
|("Small Singer Screwdriver" refers to the tiny silver-handled one that you use to adjust your bobbin tension.)|
|This brush-cap screw is made from Bakelite, not metal. Do not overtighten or it will crack!|
|You can assume symmetry here for the unlabeled screws.|
Go on to Screwdrivers, Part 5: Bits That Fit a 201-2