Here is an older style buttonholer and a hemstitcher
The screws show the funny shaped one that comes with the hemstitcher and some of the buttonholers, and the fatter round one that is standard with the plastic case buttonholers - and some of old style buttonholers.
Either screw will work with the buttonholer - but the hemstitcher requires the funny shaped screw.
So if you need one for the hemstithcer - you can pay through the nose for one.
http://www.ebay.com/itm/Singer-sewin...item3a81fa75bf
Or find an older buttonholer that is going cheap (there are plenty of overpriced ones on ebay) With shipping, you may end up paying the same, but this way you get a free buttonholer, with box and instructions!
http://www.ebay.com/itm/VINTAGE-SING...item3a8857b201
BTW- if someone needs just the hemstitcher screw - I've ended up with several extra.