This appears to be a Mizar Shinsei astro-photography adapter for a 35mm SLR camera. It’s used to connect a camera body to a telescope for astrophotography.

Based on the answers, it likely works as a telescope camera adapter and may also support eyepiece projection, where an eyepiece is fitted into the adapter to increase magnification for photographing the moon or planets.

The interchangeable ring marked “P” is probably part of the mounting system, and in your example the rings seem to be for an M42 camera mount. In other words, the tube adapts between the telescope side and a camera-specific mount ring on the camera side.

So this is not a general photographic teleconverter; it’s a specialized telescope-to-camera adapter for 35mm SLR astrophotography.