1. Norlin took over Gibson in December 1969, although there seems to be some disagreement over exactly when the Norlin era began.
2. During the Norlin era various changes were made in the construction of most Gibson models. The Norlin era acoustic instruments are generally regarded as inferior. Many would say the same about Norlin era electrics, but they did actually make some good electric guitars during the Norlin era. They also introduced a lot of new instruments most of which did not do well. Examples include the L6-S, Marauder, Sonex etc
3. I'm not sure that too many people would have a preference for a Norlin era Gibson over another. He probably didn't go out to specifically get a Norlin era SG. I have a Norlin era Les Paul Deluxe and it's a great guitar, but I didn't go out looking specifically for a Norlin era guitar. I just bought it because I liked it.
4. No. I think maybe you are trying to read too much into his choice of guitar. It probably has more to do with his age. He probably bought it in the 70's or 80's new when all the new Gibsons in guitar stores in both the USA and UK etc were Norlin Gibsons.