"Johnny say Pa" always stumped me. Thanks for clearing that up!
From context "sesame-afterfact" is "accessory after the fact." Perhaps "accessory" becomes "sesame" in future rhyming slang? Or perhaps it's a play on "open sesame" -- an accessory after the fact facilitates the escape of criminals, and "open sesame" denotes opening a door (through which one might escape). The latter seems a bit farfetched, though.