RMS is standard thread for most microscope objectives. But some microscopes have other thread (LOMO Epi objectives have 27mm mount thread). So, if you want to use your RMS/DIN objectives on a microscope with M27 mount, you should use such adapter.
Take note that this adapter preserves parfocal height of the objective - it remains 33mm or 45mm, like it was originally. Only restriction is that the objective should have rear diameter of no more than 24.5mm to be able to screw into this adapter. If you objective is larger, you need other adapter type (flat or with flat flange).
These are brand new adapters made of durable bronze. I'm also selling old stock of used original LOMO adapters at bit lower price while stock lasts.