AT-40, "The 70's" is fascinating radio, but brings up a few problematic issues. First, most stations did not play the Billboard Chart in its entirety, with the possible exception of WABC in New York, who always was quite anal about playing just the hits off the Hot100. The norm among the top40's around the country was more of a cherry-picking philosophy that played only the hits that were perceived as being the real hits in their local market. So when one listens to Casey's countdown, you would likely hear hits that never were played in your local market. (Some markets even deleted them as the countdown played!). Just another footnote in Top40 history, but an important one.
Beyond that, these shows are historic and interesting. Casey's description of these tunes were usually quite brief, with perhaps a quick rundown of their past history and where they were from, but that was about it. (Tight formatics, even then!). Even more interesting was that it didn't matter how the songs were mixed. If three R&B hits were next to each other in the countdown, so be it. Today, of course PD's would be trying to rearrange the sound codes! A different, and perhaps, simpler era.