The Pentax wasn't the first Japanese SLR with a pentaprism; the Miranda T, shown at left in an image graciously placed in the public domain by Jan von Erpecom on Wikimedia Commons (which I have subsequently cropped somewhat), was the first, coming out in 1955.
The Pentax, shown at right, arrived in 1957.
But it was 1959 that was a banner year for the Japanese camera industry; in that year, three Japanese companies debuted 35mm SLR cameras; Canon came out with the Canonflex, shown at left, Yashica came out with their Pentamatic, pictured at right... and Nikon, in April of that year, introduced the legendary Nikon F camera, shown below, which soon became the preferred choice of professional photojournalists.
And, in 1962, they introduced a pentaprism finder with a light meter built in, making the camera what was called the "Nikon F Photomic":
As it happens, Exakta was there first (at least partly, as we shall see); they offered a pentaprism finder with a built in meter for the Exakta VX IIa back in 1958. However, their light meter took its reading through a window in the front of the pentaprism enclosure, not from the light going through the pentaprism, so it did not offer through the lens metering.
Eventually, Exakta would make good that lack, with the Examat, but that would only arrive in 1969.
Pictured at left is a Nikon S-2 rangefinder camera. This camera preceded the Nikon F, and it was advertised as "the Fastest Handling '35' in the field", certainly an attribute of value to news photographers. Its predecessor, the earlier Nikon S, was widely used by news photographers covering the Korean War, and their familiarity with this Nikon product is likely to have contributed to the merits of the Nikon F camera being quickly recognized.
Oh, and I finally located an image of the actual Nikon S, which is shown at right; the advertisement in which I saw it simply referred to it as "The Nikon camera", as it was the first and only model of camera from Nikon that was available in the United States.
Shown at right is the Nikon F2, which came out in 1971 as the successor to the Nikon F, and
shown at right is the Nikon F3, the successor to the Nikon F2. This camera came out in 1980. The Nikon F, the Nikon F2, and the Nikon F3 all have removable viewfinders. This allowed Nikon F and Nikon F2 cameras to be upgraded to include automatic exposure control, and it also allowed the ground glass screen to be exchanged for ones having different designs in which the features for assisting in focusing the camera were different.
This was also true of the Nikon F4 (1988) and Nikon F5 (1996). The Nikon F6, from 2004, the final film camera in this line, did not have a removable pentaprism, but the focusing screens were still interchangeable.
The characteristic of having a removable pentaprism was shared with the Exacta cameras for which pentaprisms were available, and with the Praktica fx-2, and fx-3, and the later Praktica VLC, VLC 2, and VLC 3 as well as the Praktina fx and the Praktina IIa, some of which we met earlier.
Also, the impossibly rare Zunowflex camera from 1958 had a removable pentaprism. While the Pentacon F from 1956 introduced the internally-coupled automatic diaphragm, and the Asahiflex II rom 1954 introduced the instant-return mirror, these were still not common features in 1958, but the Zunowflex included them both. Unfortunately, the camera was plagued by quality control issues which were what led to its being discontinued.
But Nikon wasn't the only later camera maker to offer this feature in their most versatile cameras. Interchangeable viefinders were also a characteristic of the Canon F-1 (and also the later New Canon F-1), the Minolta XK (or the Minolta XM outside the United States), and the Pentax LX. Also, many Miranda SLR cameras had removable viewfinders. And so did the Alps Ambiflex, which also used a leaf shutter instead of a focal plane shutter.
Pictured below are the Praktica VLC, the Pentax LX, the New Canon F-1, and the Minolta XK from among the cameras having this elite feature.
This feature was important enough that it was also sometimes mentioned in advertising for cameras. Here, for example, is an image from an advertisement for the Praktisix camera, a medium format SLR made by Pentacon, the makers of the Praktica line of 35mm SLRs:
showing the camera, some lenses available for it, the pentaprism finder (with the waist-level finder on the camera instead), and several focusing screens.
And here's a picture from an advertisement for the Nikon F,
which just shows that choices are available for the focusing screen and finder. (The wide assortment of Nikon lenses, and their quality, was also mentioned in an advertisement for the Nikkorex F camera that appeared in the same issue of the same magazine as the advertisement this image came from, as it happens: that advertisement is shown at right.)
Pictured at left is the Nikkormat FT from 1965. As an advertisement for this camera notes, it was "Developed by Nikon, built by Nikon, and designed by Nikon for use with Nikon lenses". But then, what else would you expect? It included a through-the-lens metering system, and was at a lower price than the professional F-series cameras from Nikon.
I mean, who else would you expect to be making Nikon cameras? Mamiya? Well, back in 1962, when Nikon first decided that it should be expanding the market for their lenses by offering a less expensive alternative to their professional Nikon F camera that could also use them, while they wanted to get their feet wet, setting up new manufacturing facilities that were oriented towards controlling costs while still maintaining quality - but not to the same uncompromising level as with their professional cameras - seemed like too large a step to take right away.
And so they commissioned Mamiya to make the Nikkorex F camera for them to their specifications.
Later on, after Nikon discontinued the Nikkorex F, Mamiya was still able to make the same camera, after modifying its external appearance somewhat, for sale as the Ricoh Singlex camera and the Sears SL11 camera. Both of these cameras did have the Nikon F lens mount, so Mamiya had retained the rights to use that.
Canon's first SLR, the Canonflex, illustrated above, did not use the M42 screw mount that many other early cameras favored. Instead, its mount was called the R-mount; instead of being a bayonet mount, like that of many other cameras that eschewed the screw mount, it was classed as a breech-lock mount.
However, in 1964, with the Canon FX camera, pictured above, Canon introduced a replacement for the R-mount, the FL-mount.
In 1971, the FD-mount was introduced, but this mount was upwards-compatible; FL-mount lenses would still fit on cameras with the newer FD-mount.
The newer mount was also a breech-lock mount.