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).

Quote Originally Posted by bobwysiwyg View Post
I picked up a non-Nikon bellows unit off eBay [...]
It says "Nippon Kogaku" on the thing, which, i believe, together with the famous Nikon "F", indicates that it is an original Nikon unit.