You had to have used an Extaka (or Exa) in the early '50s like I did a lot to really appreciate what Pentax (then, Asahi Optical) did! You took a picture and everything went black, The Asahiflex had limitations, but was the first SLR to give instant mirror return (and led to the later Pentax SLRs). I didn't have an Ashaiflex, but I had all the screwmount Pentaxes, the first two Spotamatic models, and of course a K1000.
Asahi Pentax (called Sears Tower 26 in USA): first SLR with right-handed rapid-wind thumb lever, first fold-out film rewind crank, first microprism focusing aid. First Asahi SLR with M42 screw mount. Established the "modern" control layout of the 35mm SLR. Well-integrated focal-plane shutter, instant return mirror and pentaprism design. Most people have never heard of this camera.
Nikon's 'F' model was introduced in April 1959.
SLR cameras changed rapidly in the '50s (after WWII).