The oldest example of the dual rear strap buttons that I know of is the Ampeg AEB-1 Scroll Basses of 1966. They have a strange tailpiece assembly that hangs off the back end of the body on a pair of steel posts. They machined the strap buttons right onto the ends of the two posts. So there were two of them. Customers liked them. So, when Ampeg came out with the AMB-1 model Scroll Bass in 1967, it didn't have the overhanging tailpiece, but they went ahead and put two conventional strap buttons on the back end of the body, spaced about 2 1/2" apart. And a tradition was born!
I've been carrying on that tradition, putting the dual rear strap buttons on all of my new-generation Scroll Basses. They do protect the back end of the body, and help keep it from tipping over when leaned up against something. I generally put my strap on the upper button.
Does anyone know of any earlier examples of dual rear strap buttons, prior to 1966? I don't remember any.