The Chess Mysteries of Sherlock Holmes, 1979
3+1. What did Black just play?
Solution: Black did move his King (what else?) From a7 (where else?)
On a7 he was under a seemingly impossible check by wBg1. In fact this check was a discovered check, and the discoverer has now disappeared. It has disappeared because it has just been captured by the black King. Ergo it was a wN.
Last move was -1. Ka7xNa8, after Wh. played Nb6-(x)a8. A very economical Type B problem.