michael hagerty said:Total commercial time in an hour was most likely 18 minutes (the NAB standard for the time).
And there you have hit on a key point.
Stations in the 50's and 60's, if successful, ran as near 18 minutes as possible.
The NAB code, eliminated as being a form of collusion, recommended 18 minutes.
But the FCC went into fighting mode over 18+ minute hours. And that's because renewals came every three years, and part of the process was submitting logs for a composite week... a Monday from one year and month, a Tuesday from a different year or at least month, and so on to complete a 7-day week. If any hour had over 18 minutes of commercials, you pretty much had to submit an explanation or the FCC would ask for one. And that, in the era of cross filings, was a dangerous situation to be in.
The lower spot loads of FM following the 1967 simulcast ban came part out of synergistic strategy and part out of the fact that FMs couldn't sell 18 minutes of anything. Some, like the Shulke and Bonneville stations, insisted on 8 minutes an hour... but as time went on, FM became as commercialized as the 50's and 60's AMs and we are back around 14 to 18 minutes depending on the format.
Still, most music FMs have about 25% fewer spots than music AMs of the 60's did.