Question on Singer part (feed dog cover plate???)
 

The item circled in the photo below looks like a feed dog cover plate. It was included in the sale of a Singer model 15-91. Since it is photographed with the buttonhole attachment, is this part used with the buttonhole attachment?

I've got quite a few Singer buttonhole attachments here but I've never used them & none of them included the part that is circled in the photo.

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You have a complete buttonhole kit for a short shank machine. They came with 5 cams for different sized buttonholes, (extra sizes came separately) the cover plate to cover the feed dogs, the screw to attach the buttonholer, the instruction booklet and the buttonholer in the green case. The maroon/rose coloured cases held buttonholers for slant shank machines (301 for example). If you can drop your feed dogs, you don't need to use the cover plate. The feed dogs must be disengaged (covered or dropped) for the buttonhole attachment to work. This set looks to be in great shape and is pre 1960. About then or just a little later, the buttonholers were made of beige plastic and the cams were plastic too. I use this ancient old buttonholer with my Janome 6500 (yes it fits and works) because it makes much better buttonholes than the automatic ones built into the machine.