Conversation I've had Bill, indicate they were using a Sherwinn-Williams product called 'Polane' back then, though I'm not sure exactly when that stuff ran out. The short version is; as the formula for it changed, it bacame unsuitable, and they switched to the poly finish still in use today. I thought about 1979-80... happy if not eager to be corrected on the facts.
As I recall, "Big Irv" is a pretty close neighbor to my Series I, #77-621, (maybe a few dozen numbers newer?) which is also finished in Polane. Mine shows some checking, but not enough to consider a refin yet. Oil finish is a PITB, but would look mighty fine on that mighty guitar!