The first time I ever heard of a full length down rail combined with a rolled bottom was around 1970. Mike Cundith was using this arrangement on his early twin fins. He swore by it, saying it was "the best of both."
In 1972, I watched Donald Takayama shape a board for my college roommate, and he was using the same bottom/rail idea.
Over the years Greg Liddle has also shaped a number of boards -- mostly thrusters -- with the same design premise.
The board pictured above was made for Ryan Conder in 2013. Ryan can ride anything, so any endorsement he makes has to be qualified...that said, he likes it!
You can see from the cut file images how the bottom is designed. The board I'd like to try this idea on is a classic Lis Fish. Might be a real sleeper...