When did Rambow use the 5/16-18 joint pin?
Certainly none that I'm aware of, though there "could" be exceptions, or someone over the years could have replaced a pin with another. 5/16 X 18 was generally more modern, with Paradise, Palmer, possibly Fedunka and Karl Mayer, maybe some others. The typical Rambow pin was 5/16 X 14, and fairly shortly extruded from the joint. Here is an example of a typical Rambow joint pin.