Is there a difference between a slippery sam and a locking sliding shell? I have seen some people refer to them seemingly interchangeably, but they are listed as separate items on certain websites selling them.

They can be different in that most basically, the regular slippery sam doesn't lock onto the coin. The locking model does, allowing for some interesting added manipulation and effect. That's pretty much all there is to it.

Slippery Sam was (is?) a coin set, enabling a stunning simultaneous change of two coins shown back and front, one in each hand. It is not the same as Steve Dusheck's Ultimate Copper Silver Transpo gaff or the more recent sliding and locking shell work. We'd have to go into secret sessions or PM for more details on the gaffs involved.