These annotated images of M33, by van Maanen 1922(1) and Lundmark 1927(2) aren't aligned precisely enough, and the respective plate constants[*] are probably different, so the comparator produces some false motions. Even so, you can inspect each researcher's version of the internal motions in M33, including those of the 24 comparison stars.

[*] Registering spherical information on a flat plane, i.e., photographic plates, usually introduces radial distortions. These distortions tend to be more pronounced toward the edges of the plates and especially toward their corners. Plate constants are used by astronomers to compute truer directions to stars (on the celestial sphere) based on their image positions on a given photographic plate. Each telescope (and each observing position for a given telescope) results in a unique set of plate constants.

Lundmark's velocity scale is about 30 percent larger than van Maanen's.

(2) Lundmark, Knut, "Studies of Anagalactic Nebulae," Nova Acta Regiae Societatis Scientiarum Upsaliensis, Volumen Extra Ordinem Editum 1927, Plate III. (Thanks to William Keel for kindly furnishing a copy of this article.)