There's two versions of that broadcast.
The original version had Casey doing his original countdown in his normal fashion. However, when news broke regarding Lennon's death, Casey rushed to the studio and did a fitting tribute to John. However, the show had already shipped to stations, so Watermark sent two replacement discs. On the original broadcast, Casey answered a question regarding dead rock artists, but it was omitted in the new broadcast. The replacement disc also had Casey's tribute to John.
Apparently, Sirius XM played the original broadcast prior to Lennon's shooting. Fortunately, a few years ago, I was able to hear Casey's tribute on a terrestrial radio version of classic AT40.