I don't know why this didn't occur to me before. I did a little more Googling and I found this trade ad, which must have run sometime in 1973, probably in
Broadcasting magazine. Note this doesn't specify for five-a-week strip or airing once or twice a week.
Not sold in the Chicago market (or Los Angeles), but it was originally on WTOP in Washington, D.C. WNBC, WCAU, WWJ in Detroit (now WDIV), KPIX in San Francisco, WTAE in Pittsburgh all confirmed.
Some of these markets may only have had four or five commercial stations, and there wasn't necessarily room for every syndicated program. That wasn't the case in New York, where I grew up (we had six stations, and the only syndicated programs in the 1970s I don't remember ever seeing were the Garagiola
Sale of the Century and the 1973 version of
It Pays to Be Ignorant.)