This is the big brother to my Graflex Junior. The "R.B." means "revolving back," so you could turn the sheet film holder in the back to shoot in both portrait and landscape orientations without having to remove the camera from a tripod. And actually, since this camera has a waist-level finder (you'd hold the camera around waist-level, and peer down that big smoke-stack hood on top), having the revolving back also meant you didn't have to hold it sideways.
This guy is big: you can't tell the scale from this photo. It has the same contraption of a focal-plane shutter and mirror combo as the Junior; it takes some time to learn the moves necessary to make it all work: flip the the mirror down, compose, set the lens, set the shutter speed, flip the lens up, trip shutter, repeat.
This one is a Series B, which were made between the wars. After World War II they came out with the Series D and Super D, which had a different lens focusing mechanism. But they were dinosaurs by then. Their real value lay in the fact that you were composing the image through the taking lens; but German and Japanese SLR cameras showed up, and they were smaller, lighter, more compact and offered far more flexibility.
I first saw one of these in an old Abbott & Costello movie when I was young. I don't remember which movie it was—I think it was one of really bad ones where they went to the moon. But the beginning of the movie had the two of them wandering around in front of a building, offering to photograph passers-by, and each of them had one of these cameras. You can also see it in an old Harold Lloyd talkie from the 30s called The Milky Way, about 1/3rd of the way in when the press photographers show up to meet the milkman who decked the boxing champ. A bunch of them have this camera.
Another camera that's seen better days, but hey. I saw somewhere where I can get a replacement piece of leather for the front (the same piece of leather is missing on my Junior). And the optics desperately need to be cleaned, and the focusing knob was gone (someone stuck a big old generic knob on the side as a replacement, but it's obviously wrong (you can see the naked arbor sticking out in the extreme lower left corner on the photo). I got a piece of stock brass and turned it on the lathe to approximate it, so while it's not authentic, it's close enough to do the job.
Camera manual: Orphan Cameras.com