It's an 18 size movement, 17 jewels, made about 1898.
There is a sort of U/V like symbol as a serial number prefix on the secondary serial number stamps.
Here is that mark under the balance cock.
The problem is that wear and damage in the plate around the screw that the piece turns on causes the screw to go down too tight and bind the moving part. The head of the screw it supposed to sit on a lip, visible here, leaving the part free.
The washer is left over from changing a balance staff. It is worth hanging on to those!
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