Yes, that’s the micrometer. Imagine taking a long exposure of a comet (30 mins to an hour) using offset guiding on a star. The micrometer was rotated to the appropriate PA and then used to get the correct offset rate. If you messed up you wouldn’t know until you developed the single film frame sometime later. Imaging was really hard work in those days! Have a look at this example and read the caption to see what I mean.
British Astronomical Association
Supporting amateur astronomers since 1890