"..made on 1900 at Birmingham by Williams James Dingley.."
Where do you see this information? How do you know that it is a fob watch?
The chain looks to be newer than the piece. A straight piece of chain would be used for a fob watch, with one end fixed to the watch, and the other end having a straight piece, rather like cuff link, to connect to a button hole.
The watch would be removed from the pocket and held in the hand to be seen. This item looks to be too elaborate in shape to be intended for holding in the hand.
If it has a watch face behind the cover, it would be very small.....too small to see easily, and too small proportional to the size of the piece.
I would hesitate to say it is a fob watch..........and I know nothing about watches, offering some thoughts to work with.