Thank you for looking up the year, Olie.
No question that the tone and feel of an instrument are most important, but the quality of the workmanship plays a big role in desirability and, to me, value.
In the mid 70's, I visited a large music store in Sydney that had on display a number of Gibson acoustics. I was disappointed when noticing the decline is quality: the worst feature was the way in which the pickguard was affixed: instead of being glued and the edges smoothed, the pickguard on all of the top models was lumberjack thick and screwed to the soundboard. Some of the slotted flathead screws were not flush with the top; so you could catch a fingertip if rubbing against the edge. The overall appearance of the Gibson's was cheaper. The finesse in my SJ to which I am acquainted was missing.
A friend recently bought an ES-335, and the quality looked good. So I figure the period of sloppy work at the factory ended.