I don't think the original peacock was regularly used that late. I believe in its last years (through the end of 1975) it was only used on specials. The (first) "new" peacock was introduced in 1979.
I remember circa 1973-74 when WFLD-Channel 32 in Chicago ran Popeye and the Little Rascals, they would superimpose the words "Black and white telecast" at the beginning for a few seconds.