On the American Top 40 show dated March 25, 1972, Casey Kasem took the weekend off. His substitute host: Dick Clark. As Clark explained on the air, Casey was late getting back from a trip to Muscle Shoals, Alabama, and asked Clark if he could fill in.

In the spring of 1972, AT40 had been on the air for less than two years. It was by no means the institution it would become; neither was Casey as famous as he was going to become. Clark, on the other hand, was as big as anybody in American media has ever been. And he offered the producers of AT40 a suggestion that changed the show forever.

From the birth of AT40 in 1970, Casey and crew recorded each segment of the show live in real time. So if Casey flubbed a line, or there was some sort of technical problem, it was necessary for them to go back and re-record the whole segment, redeliver the lines that been OK the first time, sit through the songs again, and so on. According to Rob Durkee, who’s written extensively about AT40 and posted a tribute to Clark at his AT40 forum, when Clark showed up to record his show and found out how it was done, he offered a suggestion: why not record Casey’s intros and outros separately and then edit them into the show?

The producers thought it was a mighty fine idea, but they weren’t ready to adopt it on the fly for Clark’s show. The March 25, 1972, edition of American Top 40 sounds like it was done the old-fashioned way. Clark often seems to be winging it from notes or ad-libbing entirely, although some parts of the show do sound scripted, and you can hear what sounds like an edit here and there. Still, the show has the ad hoc feel that was not uncommon in the early years of AT40—but which disappeared later in 1972, once Casey and his producers adopted Clark’s suggestion.

One of the geeky pleasures of the old shows is that Casey usually played the radio versions of the hits—and some of those radio versions have been lost to history. As we’ve noted before, stations are not rigorous about playing the radio versions anymore: Everybody who plays “American Pie” plays the whole 8:36 now, whereas back in the day Top 40 stations would usually have played something shorter. When I bought my 45 of “American Pie” 40 years ago, the song was split into Part 1 and Part 2, and that’s precisely what it is—the first half of the song on side 1, the second half of the song on side 2. Some stations played Part 1, but Don McLean’s record label also issued a radio version that was a different take entirely. It omits all of the slow verses, removes one of the fast ones (“Helter skelter in a summer swelter/ The birds flew off with the fallout shelter”), and has some slight instrumental differences.

And that’s the one Dick Clark played when he sat in for Casey. That segment is below. The entire Dick Clark show was posted to the AT40: the 70s Facebook group last week; stations airing the 70s repeats will be given the option to run the Clark show this weekend if they choose.