This is a position eyepiece micrometer which is a type of filar micrometer. This micrometer is made entirely of brass and has no thread. It is similar to AST0859 but complete with eyepiece plate. It has a threaded aperture for medium size eyepieces and into this is screwed a ring adapter for small eyepieces. It has two silkworm threads, intact but dirty. It is in working order.

It would probably have been used to measure the distance between stars or between nebulae, star clusters and stars by the maker's older brother, William Herschel.