Round 1960 Nikon introduced the F mount. Their first camera was the Nikon F, a beautiful constructed camera with a choice of viewfinders (eye level, waist level, several focussing screens). As a customer you could basically modify the F body to your specific need. This came with a cost of course. The F mount had a steep price tag and the accessories too.
Nikon was desperate to make the F mount a success. In order to do this Nikon was of course making a big effort to launch as quick as possible a range of suitable lenses. Lenses are complex things to make, specifically when a new mount is introduced. A lot of effort was put into this area. Availability of lenses covering a wide range would be key to drive the demand and success of this new camera.
But Nikon perhaps also understood that the F body was not reachable for every consumer. With potentially a big roadmap of new products to be introduced, Nikon eventually outsourced a camera development. Target was a far cheaper SLR camera with of course the F mount.
There are a lot of stories on the internet who and how this was arranged. In the end Mamiya constructed the camera in cooperation with Nikon. Mid 1962 this camera was introduced. Nikon however decided not to brand this camera Nikon, but introduced the name Nikkorex. Nikon did this for a reason of course: the camera should not cannibalise the sales of the Nikon F. The main purpose of this Nikkorex SLR was to increase the sales quantities of Nikkor lenses.
The Nikkorex was build around the F mount, with a fixed prism and focussing screen. A very reliable Copal shutter mechanism. It felt maybe a bit less sturdy, the construction however was certainly not bad. The internal mechanisme was actually slightly smarter constructed. Specifically the elements for the slow shutter speed. It also had an hinged door, making loading film a bit easier. In the end everything was done to reduce cost. Leaving out all the interchangeable parts of a Nikon F did do the job. The Nikkorex was 40% cheaper and still suitable for the great F mount lenses.
In the few years that this design was made 2 versions were introduced. The latter clearly showed the need to reduce production cost. Polished parts were replaced for anodised aluminium, common parts were used and the rear door with the ASA setting reminder, was replaced by a simplified door without this reminder.
Now the funny bit starts. Mamiya Sekor introduced specifically for the Nikkorex camera 2 lenses. A 35mm f2.8 and a 135 mm f2.8. They both were branded Nikkorex Lens Sekor and clearly made by Mamiya Sekor. The lenses had the F mount, but were semi automatic. Basically this means that after each shot the aperture stayed closed to the set aperture. A lever had to be turned to open up the aperture and lock it. When the customer would press the expose button, it would push a lever to free the aperture which then would stop down to the set position. For the next shot you would need to repeat this process.
It is unknown why Nikon let Mamiya also construct these two lenses. Maybe the Nikon lens roadmap was too ambitious. Very little is known about the quantities built for both lenses. The quantities must have been very low as Nikon themselves introduced in 1964 the 135mm f2.8 automatic lens (Auto Q). Fact is that these Nikkorex lenses are very rare.
The built of these Nikkorex lenses is very similar to the Nikkor Kogaku 5.8cm f1.4 lens. This Nikkor Kogaku 5.8cm lens was introduced prior to the later standard 50mm lens. The Nikkor Kogaku 5.8 cm has a big internal brass helicoid. A construction similar to the Nikkorex lens, where there is one large brass helicoid and a second aluminium to aluminum helicoid. Both can be seen on the right at below picture. Perhaps this fed the rumour that the Nikkor Kogaku 5.8cm f1.4 was outsourced. There are no further similarities though, beside the fact that both lenses are heavy as result of the amount of brass and glass.
Recently I got an old and forgotten Nikkorex 135mm lens which had completely frozen helicoids, fungus on rear and front lens group. The construction of this lens is not described on the internet (well at least I could not find it) so it was a bit of a challenge. The lens can be disassembled up to the point of a aluminium housing that (as it turned out to be) contained the two helicoids.
Frozen helicoids, specifically brass onto aluminium, are problematic. The fine screw threads creates a very large surface area. As grease overtime completely dried out, it basically glued the three parts together. Lighter fuel, WD40 did not make any difference. Auto-mechanic websites offered a possible solution: boiling in hot water. As grease often was made with base of soap, this could potentially be the solution. Indeed after 5 minutes of pouring hot water over the aluminium and brass parts. First the large brass part and soon after the much smaller aluminium helicoid were released. Nothing was damaged and the helicoids were in great shape.
Fungus was removed without leaving any trace of etched glass. Everything was assembled with great care and precision. Turns out that this is actually a fun lens to use. Even on digital. It is a hefty lens. The lens renders nicely.