Interesting Wikipedia article on early red-dot-type sights.

https://en.wikipedia.org/wiki/Reflector_sight

Short version: They were invented in 1901, were in military use in World War I, and were highly developed by World War 2, but their application to small arms was limited by battery life until the development of LED's and better batteries.

Early Grubb sight (patented in 1901), which collected ambient light for the dot:



And a Mark III reflector sight from 1943 (I assume for an aircraft gun):

 

Reflector sight is the preferred name for reflex sight but I suspect a lot of us are more familiar with the latter term.

Technically reflector or reflex sight is different from an in-tube red dot scope but the general idea of a lighted dot or reticle aiming point is much older than most people realize.

Good find, OP.

ADDED: As long as you had a light source. Batteries take care of that today but batteries get depleted. Could a battery be charged from a solar cell?