I think it was '77/'78, but I could be wrong. That is, either he was quoted as saying it then or was recounting that he originally heard it then. It's in The Secret History of Star Wars, at least the first version.

EDIT: But I don't think that necessarily steps on the toes of the evolution of the Emperor. I just added in the force lightning explanation for a new version of the scene. It doesn't necessarily mean that that's what happened in the account that Mark Hamill is speaking of. I mean, the Emperor could have shown up with dozens of armed guards, and that's what scared Obi-Wan off. Of course, I'm just speculating on that too.