I seem to have shared with geeker that 23a was FOI (and only one, for some time - 15d was second). I think I have come across it in books by Us authors?
The first (and only?) time I ever heard of 23a was in The Godfather, when Mike goes to meet Virgil Sollozzo, and Clemenza says that their reciprocal 'hostage' (to guarantee Mike's safety) is happy p.....ing. 23a.