'Unique' Swiss 'castle' hallmarks help

Hello, I have inherited an old pocket watch and after doing a lot of looking online believe it is a Swiss imported watch bearing the 0,935 purity mark and three bears indicating it dates between 1887-1907 and a small ‘B’ indicating Bienne as the Swiss assay office.

However, I cannot find any reference to the 3 ‘castle/fort’ stamps are. They appear to have letters in them. Are these maker’s marks? I thought the Swiss didn’t tend to use maker’s marks.

I am guessing the number at the bottom is a serial number.