Don...a question would be: are all M4+ quakes that occurred recorded on this list for the 1950s, 1960s, 1970s. I would not count on it. It may be that a network was installed around the late 70s or so (I don't know, but you can ask someone). While M4+s would be felt, and could have been located, they might not be precisely located...or they just might not make the list.
I'm not ruling out what you noticed being real.
Chris