The ring is a helicoidal focusing ring.
I had used both versions of the Cambo wide for decades, with the longest lens being 90mm. Eventually I wanted to use the 150mm lens on the Cambo Wide DS . Soon, I saw the need for a slight tilt capability. I designed and fabricated this assembly using a lens board for the 58mm Lens and making a hinged panel with adjustment screws and a small bellows. The hinge was scavenged from a Polaroid camera. It was a great modification giving me much greater focus control. I've since acquired a fold up 4x5 and make a lot of use of longer lenses and the Cambo seems to be neglected these days.
That's pretty cool, Werra. I bet you'll make a lot of good photos with that nice simple setup. It's a lot like a 4x10 camera I made several years ago in school. It was constructed a lot like yours. It had a very wide lens, though, and had a curved back made of laminated poster board. It included rails that the film would slide under. The camera was a "single shot" and had to be loaded in a darkroom before going out.