Hey Marty, from what I have learned in recent years and combined with my memories from the 70s I would say that frame was made in 79 (at the earliest) or the early 80s. When I was there in 78 I think they were just developing the flat brake bridge and they did not have chain tensioners yet.
As for the serial number, I was told they started over on numbers every time they changed frame design.
I will contact Claude (Masterstint) he is the Cook expert and I believe he is a member here.