Sheffield Pocket Watch

I bought this watch at an auction and have no clue what it's worth. I liked it and figured I'd just turn it into a necklace. It's open faced and keeps time perfectly after I wound it up and set it. It seems to be a clamshell back and I can't get it open so I don't know how many jewels it is. Any help would really be appreciated. I've looked all over the internet but can't find one that looks just like this one.

Fair