This mount is used on polarization microscopes as far as I know. For example, I can see it on MIN-10 microscope in my reference book (see the scan).

To work correctly, it requires special clamping mechanism (not included - should be a part of your microscope).

Such adapter allows two things - to change an objective quickly, and to center it prefcectly by means of that spring-loaded mechanism and two adjusting screws in the adapter.

Adapter thread is RMS (0.8″, 36 tpi).