The left plastic thing is identified! Thanks to SHELTIE'SMAMA, who got me searching in the right direction, I finally found that indeed it is a "Stitch Chart Holder (holds the Stitch Pattern Chart for easy viewing)" for a Singer Confidence 7640 sewing machine.
https://www.manualslib.com/manual/12...page=11#manual shows it as an accessory that comes with the machine. At
https://www.manualslib.com/manual/12...?page=8#manual shows that it goes in the handle of the machine. From there I found
https://www.createandcraft.com/gb/pp...arranty-427951 which has pictures (almost the end of slide show) of the machine with the holder in place. It also has a picture (#6), not just drawings, of accessories.
The feet that I got with that piece also has a lot of the accessories in the picture at createandcraft. Snap-on feet that have a letter on them are the same as what I have.
I looked at the walking foot I have, and some other non-standard feet, but couldn't identify the other plastic piece.
And I haven't found the buttonhole attachment that would have come with that feed dog cover plate.
Janey - Neat people never make the exciting discoveries I do.