These were in an old treadle sewing machine. There were news clippings from the 40's and even a voting ballot from 40. They are not plastic you can see lathe tuning marks and the slots are hand sawn. The ribbon was initially attached and was elastic, it is sewn on to the ribbon through one hole. The little flower embellishment with seed beads is hand made. They do not slide together. They appear to be ivory or bone. There is a bit of discoloration/stain on one in the top picture at the slot that is not a break it is something that stained the piece. I don't think it is a needle case due to the slots.
I initially thought "lace bobbin" but have never seen one in person, and the examples I have seen have never been attached together and seem to have a bit of weight to pull the string down.