Will case screws from a 16s Waltham fit an 18s Waltham?

Lorne I think the only way to go is to try them.

A 16 size Waltham 1899/08 model case screw will fit an 1892 model 18 size Waltham but can be too short. Best bet are the screws with slightly domed and polished full heads.

I knew this wasn't going to be simple. a guy has some for sale from a Waltham 16s. I will just have to ask questions