I think CBS called that a "chain break" (the :33 local cutaway in the middle of a half-hour soap. It probably went away sometime in the 1970s. An example of this is at approx. 12:18 into ATWT on 11/22/63--coming out of a network break, there's a CTN for Route 66 and a :33 station break (in this instance it's more like :37 or so of black).