I always got confused with the designations, I, III, Class A, D, etc. ;-)
David, is there a reliable list of the original 1-A Clears?
What factors made them different from the 1-B's?
The original I-A clears:
KFI, WSM, WNBC, WMAQ, KNBR, WLW, WGN, WSB, WJR, WABC, WBBM, WBAP/WFAA, WCCO, WHAS, WWL, WCBS, KDKA, WBZ, WHO, WTAM, KMOX, KSL, WHAM, WOAI, WCAU
These were originally unduplicated at night across the entire US, and had no requirement to operate directional antennas to protect any other station. (Several chose to use DAs to concentrate their signals over land instead of water or foreign soil - WBZ and WWL still do, and WNBC, WLW, WTAM and KNX experimented with DAs at various times.)
The I-Bs still enjoyed skywave protection of their signals (which would later entitle them to the same Class A status as the I-As) but were usually DA-N so as to allow several stations to share a frequency. WOR, WGY/KGO, KOA, KYW, KNX, WTIC, WBAL, WBT/KFAB are all prominent examples of I-Bs. A few of them - WGY, KOA, KNX - were far enough away from the stations with which they had mutual protection so that they could still operate ND-U. WBT is a special case because it was originally a I-A but downgraded to I-B when its then-owner CBS wanted to move KFAB in Omaha. KFAB had been synchronized to CBS-owned WBBM on 770 and then 780; downgrading WBT to I-B (and adding two more towers at night) allowed it to share the 1080/1110 channel with KFAB and got KFAB off the same channel as WBBM.
And there was a 40-year legal battle waged over WABC's I-A status. KOB in New Mexico fought to get WABC downgraded to I-B status so that it could also operate as a I-B in a region that otherwise had no powerful full-time AMs. WABC fought back, and the matter wasn't fully resolved for decades. WABC ended up as a de facto I-B, since KOB was allowed to operate at night while the court fight raged on. By the time it was resolved, even though WABC kept its nominal I-A status, the FCC had created the class II-A stations that duplicated East Coast I-A channels out west at night, and you could no longer hear the East Coast clears from coast to coast, even if they were still I-As on paper.