I thought the “. . . Not!” construction was new when Wayne & Garth used it, but here it is in the mouth of Archie Goodwin:
“We know she didn’t kill her husband. Either you thought she had and probably still do, or you killed him yourself. If the former, your feeling for her has got a smudge. If the latter, you did a swell job, not, handling it so that she gets the credit for it.”
“Death of a Demon” by Rex Stout, published 1961 as part of Homicide Trinity.