Forgive me if I'm way off base here, but going back to No Go Showboat, could that mystery vocalist be David Marks? I have no idea what he sounded like back then or if he was even in the band at that point, but he was pretty young, and it sounds like a young male voice.
I an now inclined to come full circle on this -- absent any other evidence, who else but David Marks could this be? (I don't think this was an instance of Mike Love's "mock" falsetto as was suggested in a previous post.) Unless perhaps some anonymous "session" musician was given his vocal "moment in the sun" here.....
What does give me pause, though, is this: I would have thought Jon Stebbins would have jumped on this by now (confirming this to be David's voice). (Maybe Mr. Stebbins has not been reading this thread?) The only other instance of David Marks' singing as a Beach Boy (that I know of) is his duet with Carl on "Summertime Blues" -- and it is impossible to identify distinguishing characteristics in David's voice in that instance (to compare with the vocal snippet in No Go Showboat). Does anyone know of any other (confirmed) instances of David Marks singing (background vocals or otherwise) for the BB's?