The Nydar Model 47, contrary to it’s name, was invented in 1945 by Clinton S. Davis, and made by Swain Nelson Co. This is not Mr. Davis’s only patent, as he received one for a “Photoflash Lamp Circuit” just two years later, as an individual.
The operating principle of the reflex sight is similar to aircraft mounted machine guns used during World War Two. The earliest patent referenced by Clinton in his application is UK#22,127, dated to the year 1900.
The optic was designed to fit many standard shotguns at the time, and was marketed towards hunters. It uses an adjustment plate and a mounting base to attach to a firearm.

Diagram and operating principle, from a 1947 advertisement

Patent Graphics

The reticle has the characteristic of maintaining it’s size relative to the background regardless of the distance from the optic to your eye. While making this more versatile in terms of weapon mounting, it does mean that placing the optic at roughly arms length results in a reticle that is far too large for practical purposes. I advise mounting this optic no further than 16″/400mm from where your eye will be.

Also note that these function based on ambient light. A dimly lit room or a cloudy day will render the reticle too faint to use.

The Nydar did not live and die in the 40’s however. The coating was a quickly noted issue, and improvements were made over time. As late as 1966, Swain Nelson Co. was improving and iterating on the design, even releasing models tailored for handgun shooting.

When Purchasing:

Take note of, or request photos of the lens. These optics have been known to delaminate over time, or have their prisms become dirty/cloudy. When that happens the reticle is nigh unusable.
Examples below:

You can readily find these on the second hand market for $50-150 depending on condition and complete-ness of package.