When I looked at the 11th ed. Electric Guitar Blue Book links from Gibson it extended that "dater" info serial to 1974 &1975.
From the 11ed. Electric Guitar BB: "600000S 1970, 1971, 1972, 1974, 1975". It goes on to say "During the period from 1975-1977 Gibson used a transfer that had eight digit numbers," and " When the Nashville Gibson plant was opened in 1974, it was decided that the bulk of the production of products would be run in the South; the Kalamazoo plant would produce the higher end (fancier) models in the North. Of course, many of the older guitar builders and craftsmen were still in Kalamazoo, and if they weren’t ready to change how they built guitars, then they may not have been ready to change how they numbered them! Certain guitar models built in the late 1970s can be used to demonstrate the old-style, six-digit serial numbers. It is estimated that Gibson’s Kalamazoo plant continued to use the six-digit serial numbers through 1978 and 1979., from the 11ed. Electric Guitar BB: "During the period from 1975-1977 Gibson used a transfer that had eight digit numbers,".
The 6th ed. Electric Guitar Blue Book says: "1974 and 1975 100000s, 200000s, 300000s, 400000s,500000s, 600000s, 800000s"
So it would appear that 1975, Kalamazoo or Nashville to be the best "guess"; leanin toward Kalamazoo.
Thanks guys.
I'll try to get confirmation from Gibson. I thought a mod may be poking around. I'll post a pic this afternoon.