I guess "Clear View" is the brand, although I've never heard of it. It's a T-mount, which means that it can be adapted to whatever lens mount you can find. Currently, I have a 42mm screw mount adapter, which is easy to adapt to digital.
Back in the day, these were called "cats," which stood for catadioptric and is a design for a telescope. With a telescope, the eyepiece is mounted to the rear of the lens.
The aperture has full and half stops from f/8 to f/16. There are no visible aperture blades so I'm not really sure how it does this. Maybe the aperture simply is part of the rear of the lens and cannot be seen.
Filters can be mounted in back of the rear element.
Like other long focal length lenses, it can focus past infinity to allow for barrel expansion.
It's a sharp lens, but handholding can be a real challenge.
Here are a couple of photos. If you’ve not shot a mirror lens, its major characteristic is the donut-shaped out-of-focus regions.
These were shot with a Sony A7 II.
This is two horses, although it looks like one.
This was shot with a 50mm lens so you can get an idea of the difference in the field of view.
The donut-shaped out-of-focus areas are clearly visible.
I can't recall how I came to be in possession of this lens. I don't recall buying it, and I think that maybe it came with something else that I bought.