I can recall, many years ago, someone singing a song called 'Not All There' on the BBC's music hall show 'The Good Old Days' and using that old bit of stage business where the performer has special shoes which fit into clips on the stage so that he can lean over the footlights from his ankles in an unexpected and bizarre fashion, which this chap did every time he sang the line 'Not All There'.
The only other thing I can remember is that the pay-off line was something like, 'but I'm not supposed to be all there'.
Sorry, this is just a fragment of a memory, but at least it may confirm that the song (if it's the same one) comes from music hall days, which could be a starting point for research.