I recently found a loupe with a thread pitch reticle displaying common UTS thread pitches... except that rather than 40, 48, or 56 it has a 50. What would this be used for? It doesn't appear to date from earlier than the 1960s - the housing and dioptre lens are plastic, the reticle and retaining ring are glass/metal.
Fine threads (40 to 120 TPI) are used in adjustable mountings in fields such as optics and mirrors as well as other fields with small parts : watch making etc.
More detail can be found here: Fine adjustment screw