K100D, The bellows is fairly old and I've had it for a long time. It's screw mount and I use one of the cheap Ebay mounts to the camera (it's easier and cheaper than the Pentax OEM version since the slot on the side makes it come on and off easier and infinity focus isn't an issue for this.
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The bellows has a thick metal plate at each end. To that is the camera mount end and the lens mount end. The rails are secured to the camera end of this plate. The rails on the lens end have a brace between them to keep them solid and parallel. The bottom of the plate on the camera end has the same screw threaded hole that a camera does for tripod mounting. In my case I mount a set of focus rails to this part. The mount the rails to the tripod head via QR plate.
This allows for much easier and precise focusing. The Bellows can do this as well without the rail. Just turn the knob and you can frame the shot the way you want.
If you want a couple of close up pics I can post more. The Ebay bellows you see are the same thing. There are some from Russia that sometimes come up and they extend to 290mm. With those bellows, I'd get a lens like the Takumar 105mm for this bellows and then you can get up to almost 3x lifesized. going much closer than that and lighting and other issues (purple fringing, chroma aberrations) etc come into play.
The best setup I've seen is the Novaflex similar to this one.
NOVOFLEX BELLOW 1934 FOR EXA CAMERA OR LEICA M3 LENS - eBay (item 120242571009 end time Apr-09-08 04:33:49 PDT)
This one listed would be a bit suspect since it's so old. The bellows material is a type of kraft paper with a wax surface. Over time they can crack and let in light. At almost 70 years old, this one will have issues with heavy use I bet. Plus old Leica gear was M39 not M42 like the Pentax standard. But this type of bellows is still available relatively new. The plus with this bellows is the second set of rails for focusing. Mount that to the tripod and you can slide the camera and assembly back and forth to get exact focus.
There are tons of M42 lenses around and also T mount lenses. They look the same as an M42 but the pitch is different with the threads. So you use a T mount adapter that is M42 on the other end. T mounts can also be had with K mount ends to use the lens on the camera directly. The Focal 35mm (upper right in pic with red lettering) is a T mount lens. You can see rear element sticking out. The T mount screws on around the back of that element and then it looks exactly the same as a regular M42 lens. These T mount lenses are available in numerous brands ( T Mount was the Tamron standard way back, hence the "T") So you can often find these lenses for $20 or so.
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