Actually (according to Al, anyway), they did that bit only once, and it was on Letterman.
I should know better than to doubt you by now, but do you have a citation on that? I remember seeing that bit and I ahve pretty strong memories of seeing the Frankin and Davis Show in that last half hour of the old SNL. But now that you mention it, it does sound more like a bit theywould do somewhere else. I'll do some checking.
OK, that was quick. I just found a clip of Franken talking about it on Letterman on the 20th, and you are right about the time they did it--it was Letterman, not SNL. But now the thing that blows me away is that I told him my joke on the 15th and here he is talking about it on the 20th! Can that really be jsut a coincidence or did I put the idea in his head? Or at least move it from the back to the front?
That's the only reason I knew -- I happened to catch Letterman last week. I also remember the original bit, which must have been from around 1985.
My favorite Franken and Davis routine from that era was the one where they appear on stage and Al has a large bandage on his head. Tom explains that he had massive head trauma (or something), but was determined to soldier on with the act, even though he was suffering bouts of disorientation. They start out with an oldie:
Al: Did you hear about the Rabbi who didn't charge fees for circumcisions?
Al: He only took tips.
Then they launched into a series of jokes, but each time, just as Al was to deliver the punchline, he'd get confused and say "He only took tips."