Nippon Koken/Nihon Optical
An interesting company, with a complicated history. There are three cameras associated with it, two "Nikkenflexes" and a "Nikkoflex". The comprehensive story is told by the German website http://www.lausch.com/ersterteil.htm, which tells how Nippon Kogaku (the famous Nikon) developed a prototype Rolleiflex copy called the Nikoflex in 1946, but had troubles obtaining reliable shutters, eventually abandoning it in 1948, partly because its 35mm camera was beginning to sell seriously. In 1950, Nihon Optical Research (which I think is probably synonymous with Nippon Koken) launched its Nikkoflex (note the additional "k"). Nikon seems to have objected to use of a similar name, despite its withdrawal from TLRs, and Nihon Optical eventually agreed to change the name to Nikkenflex - but not until perhaps 1,000 cameras had already been built. The first Nikkenflex, therefore, was simply a badge change from the Nikkoflex shown below.
McKeown lists this first model (with a space in the middle incorrectly, as the nameplate shows) as "Nikko Flex" (1950). He also has two Nikkenflexes (none of these three are pictured in McKeown), one with a similar description to the Nikkoflex (and also dated to 1950) and one very different with Rolleicord-style knob-focus (1953). Sugiyama pictures the Nikkoflex and later Nikkenflex only. So to clarify, the three basic models are:
o The original geared-lens Nikkoflex
o The same model with the nameplate changed to "Nikkenflex"
o A later Rolleicord-clone Nikkenflex
In my collection (below), I have three cameras - the original Nikko and two of the later Nikkens.
I have been unable to unearth any more detail on the company, although one website says that a Nippon Koken Systems Co. was in recent existence in Tokyo, whether or not related. I am certainly interested if anyone has more information, or indeed a Nikkenflex to sell! Incidentally, a week after I bought this another one turned up on eBay, but the front lens panel is black-painted, so there seem to be some variants.