Based on the identifying marks mentioned, this lens was most likely made by Cima, a Japanese third-party lens manufacturer. The diamond-style aperture/branding detail is a strong clue, and similar markings appeared on Cima-made lenses sold under names like Cimko and Chinon.

It may also have been a rebrand supplied through an importer or wholesaler, possibly related to the Prinz naming family, since names like Rexagon and Rexatar were also used on rebadged Japanese lenses.

The closest match suggested by the community is that your Auto Rexar is essentially the same lens as a Cima-made 28mm f/2.8 sold under the Super-Paragon name. So while "Auto Rexar" itself is obscure, it likely isn’t a unique optical design—just a less common badge on a lens sold under several brands.

In short: likely Cima-made, Japanese, third-party, and probably equivalent to a rebadged 28mm f/2.8 design seen under other names.