Bellows Unit 1
This bellows was designed to be used with Nikon's rangefinder cameras. It uses the S-mount.
There is a rear attachment that has a reflex mirror housing. Without the reflex mirror, focusing with the bellows will not be possible.
They were not produced in a large volume, so are rare and difficult to find. Production was from late 1957 to 1959. I was unable to find a listing for one on eBay.
Bellows Unit Model II
This is similar to the bellows unit 1, but uses the Nikon F-mount. There are considerably more lenses that can be used with this and later bellows.
The Nikon BR-1 extension tube allows the 135mm f/4 lens from the Model I bellows to be used. These as well as the bellows itself can be found for sale on eBay.
It is also the first bellows that has a slide copier that can be attached.
While the bellows is still usable, unless you're intending to use it with vintage cameras, later bellows are going to be easier to use.
The PB-3 is interesting because of its' single rail design with a octagonal bellows. It was intended to be an economical alternative to the PB-4.
Checking eBay showed a couple for sale, but they seemed overpriced for what you get. A PB-4 or PB-6 would be a better choice.
Not being able to more the rear standard is a problem when reversing lenses or using other techniques to get high magnification.
Using it with a Nikon digital camera, or adapted mirrorless camera, will be a problem as well. There is not focus on the bellows so balancing the weight on a tripod will be impossible.
The Nikon PB-4 was made for professionals and is my personal favorite of all the Nikon bellows. While it is the only Nikon bellows capable of tilt/shift adjustments, that is not why it is my favorite.
Unlike all of the previous bellows released, both of the standards on the PB-4 are adjustable. This is important for macro photography as working distances get small when going beyond 1:1 magnification.
The front standard can be moved all the way forward. Doing this helps reduce problems from the rails interfering with the subject. Then, the rear standard can be moved to adjust the level of magnification.
Depending on the camera and lens used, the bellows could end up being back or front heavy. The built in focus rail makes it easy to balance the weight of the bellows on a tripod or motorized focus rail.
The PB-5 is another "economical option" when released. They are widely available on eBay and other used gear sellers.
To reduce the cost of the bellows, the rear standard does not move and there is no built in focus rail.
I had one for a short time but got rid of it because it was impossible to use on a tripod. This made it useless for macro photography.
The bellows can be a good choice when paired with the PS-5 slide copier. With the slide copier attached, it can easily be used on a table.
The PB-6 is another professional bellows and the last bellows model made by Nikon. Like the PB-4, both standards move and it has a built in focus rail.
Differences from the PB-4 are a slightly lower build quality, single rail design, and no tilt/shift movements. It is still an excellent choice for macro photography.