This monogrammer is Part Number 171174 while the Part Number on the box is 171256. The box says for use with the 750 series. (Note: Box and original monogrammer were probably separated.)
Look here for a picture of the box.
Following Compliments of Sharon Reese:
The Singer monogrammer for slant needle machines monograms letters about 3/8" tall. A complete attachment includes: letter placement guide, all 26 letters, manual, throatplate (or feed cover plate), and thumb screw for the attachment. In the third picture, the manual is from 1969, and the throatplate is for a Touch and Sew 750.
You must use a special feed cover plate for this attachment. Simply raising the throatplate or using a buttonholer feeddog cover will not work. In the last picture, the top cover (507661) replaces throatplates for Touch and Sews zigzag machines with magnetic throatplates (for example 750's). The second cover (161825) replaces throatplates for Touch and Sews and Slant-o-matic (the 401, 403, 500, and 503) zigzag machines with elevator throatplates. The bottom cover (86748) covers the throatplate of vertical needle zigzag and isn't the same as a feeddog cover for a buttonholer, even though it looks like one.