What's the third shaker for?!

https://nowiknow.com/the-mystery-of-the-third-shaker/

TL;DR: There's a unknown 3rd shaker in many 19th century table settings and no one really knows what went in it as far as I can tell. There's some indication it might have been for dried mustard powder but that's the only guess anyone has and it wasn't that popular as a condiment as far as anyone can tell outside of the tiny references in late 19th century catalogs for the 3 shaker sets including a 'mustard bottle'.