[Yashinoff]

I found this to be pretty accurate history of the SLR -> Early History of Single Lens Reflex (SLR) camera

Keep in mind that arguably the single most important development in 35mm SLRs was Asahi Pentax instant return mirror. Prior to this the viewfinders would blackout until next frame advance or some time until the mirror would drop after firing.

I would argue that the instant return mirror is not especially useful unless used in conjunction with an automatic diaphragm. It's nice that the mirror drops back down, but if the lens is still stopped down, then the "instant" part of the equation is rather less useful. The Minolta SR2 and the first "automatic" lenses for the Pentax in 1958 were unfortunately only semi-automatic, the diaphragm did not reopen automatically, and so while Minolta and Pentax both had instant return mirrors, their system wasn't quite as useful as the Nikon F (or Miranda C) of 1959. I find it odd that Asahi did not adopt automatic lenses from the start with the Pentax, since the Praktica, Edixa, Pentacon/Contax had already applied them to the M42 mount since 1954/55. Pentax was obviously aware of what the Germans were doing, so I have to wonder what stalled them on it.