Don Davison developed the Safety Rappel Cam while he was Chairman of the Safety and Techniques Committee of the NSS, and published his design in the August 1976 NSS News. As far as I know, I am the only other person that ever built one. I don't use a rappel safety myself, for a number of reasons that may or may not apply to other people. Everyone must make their own choice.
Basically, the Safety Rappel Cam is a modified Gibbs Ascender. Shock cord is added to cause the ascender to close (there were no spring-loaded Gibbs at the time) and a metal catch was fabricated to keep the cam open. The catch fit into notches filed into the shell, and a thin cord with a clip attached was clipped to the caver's chest. If the caver fell backward, the cord pulled the catch free, and the cam closed.
The Safety Rappel Cam was tied to the caver's harness with the goldline spliced to the cam. In this manner, the Safety Rappel Cam is independent of the seat maillon.