[Spazoid1965]

After extension/magnification is set, you focus by moving the entire set up back or forth, i.e. varying only one parameter - lens to subject distance.
Not by using the focusing ring on the lens, which changes both lens to subject and lens to film distances, and with it magnification (and exposure correction factor).



It says "Nippon Kogaku" on the thing, which, i believe, together with the famous Nikon "F", indicates that it is an original Nikon unit.

Yes this is an original NIKON product. I have the same bellow unit. Have you tried mounting this on your camera yet? This bellow unit is "NON-AI". I believe both of the cameras you mentioned are "AI". Make sure this unit does not damage the apeture indexing pin. I do not have either of these cameras so I cannot tell you for sure if it is safe to mount. I know it is safe on my "NON-AI" Nikkormat FT2. But on my DSLR I must use a short "AI" extension tube between the camera and bellow unit. Mainly becuase this unit will not clear the battery compartment. And I'm not sure if it will clear the index pin.

The engraving on the rails is not for the 50mm lens at all. The 50mm lens will be useless at any extension greateter than about 100mm. As the focusing distance will be inside the lens. This unit you have will extend to approximately 150mm ( 6 inches). Most bellow will only extend to 4". These engraving marks are for the Bellows-Nikkor-P 105mm f/4.0 and the Bellows-Nikkor 135mm f/4.0. Both of these lenses have no focusing ring. As focusing with bellows is done by moving the entire camera and bellows to and from the subject. Other lenses can be used on this bellow assymbly, but the engraved markings will not apply. I did a silly little expierement with this bellow unit a while back if you want to se the results. Here's the link.
http://tinyurl.com/37gaxvt
If you have any question just drop a line this way.