At sometime along the way, Japanese camera manufacturer Mamiya bought some rights from Graflex. They created a workhorse camera that ruled in studios for years - the RB67 Professional. In 1974, the camera was updated and became the RB67 Pro-S, then in 1990 a few more changes and it's the RB67 Pro-SD.
One of the keys to the camera's success was its modular concept - the 35mm idea carried over to a camera that shoots 120/220 film. Part of the modular concept was the use of the Graflock back - from Graflex. It permits the use of interchangeable backs - roll film usually, but sheet film, too, with Grafmatics and regular sheet film holders (if you could find them).
I, being contrary, use an RB67 as a field camera. I consider it lightweight - at least in comparison to my Orbit 8x10.
Mamiya posted the manual for the Pro-S on the Internet - you can download it here (pdf).
Photos on this site taken with an RB67