830 pure from my research. Many of these have had a new clasp attached and have been turned into bracelets. You can find examples on the internet turned into bracelets.
The key to this example is the double dagger hallmark you'll see on the back of the horse part at the top right which was only used between 1814 to 1865. After that a single dagger was used until 1906. Saying that this example is clearly missing something on at least one end but is unmolested so you can do the repair you think fits correctly. The other end has a crudely made ring clip to fasten onto. It shouldn't be hard or expensive to have a piece of silver added that would hold your antique silver watch from the same period or do as others have done and turn it into a bracelet.
My price is simply for the age and rarity of this nice example that luckily survived. The item "Antique 1840's Dutch Silver Watch Chain" is in sale since Monday, January 13, 2020. This item is in the category "Jewelry & Watches\Watches, Parts & Accessories\Watch Accessories\Chains & Fobs". The seller is "justneatitems" and is located in Derry, New Hampshire.This item can be shipped to United States.