Certainly at least some of them were used as watch fobs; the hardware likely attests to that (if not a watch fob it was certainly attached to
something). But the ones I have seen called "book marks" appear to me identical in construction, shape, and size—the only difference is the lack of attachment hardware on the end, which could simply be due to it coming detached. It's large enough I can imagine it getting caught on things and pulled, and I'm guessing that the ones attached to a braid were the first casualties, as opposed to the ones with the much sturdier bracket affairs.
As a book mark goes it isn't particularly functional—if you put it in beads first it makes marks on the pages, and if you put it in braid first it falls out due to the weight I tend to agree with the professor. Are there any images or documentation somewhere of them being used as either a a bookmark
or a watch fob, or is everyone simply guessing based on the construction and shape?
PS: These look very similar to the beads on this piece:
https://www.etsy.com/listing/261410107/size-110-vintage-italian-seed-beadsClaimed production late 1800s to 1920. They have he same claimed opalescent, which doesn't show up well in photos, but they do look like opal.