To expand on Jim's point about salt dried wood. It was in general use in 1975 and 1976 time period and sadly it was used more for highly figured wood as it was usually denser so took longer to dry without the short cut. There is some that shows up later as the wood was not segregated by time frame. The two best #1s I have ever owned were both 1976 salt wood guns, before I knew better.
The primary reason for the drop in QC was the HUGE expansion in production of #1s in this time frame. While serial numbers as indicated by Ruger is NOT a true and accurate indicator of total production I think it is probably close enough to use as a rough estimate..
If you look at the non prefix guns, 1967-1969 with some assembled and shipped in 1970 and 1971 and as late as 1973 they were making anywhere from 1200 to 2200 #1s a year. In 1974 they increased to approximately 7000. In 1975 they made approximately 5000. THEN in 1976 production went to roughly 23,000, unless they simply skipped HUGE blocks of serial numbers. It became harder and harder for the suppliers to meet the demand for highly figured wood, hence the shortcut with salt wood. This not only effected Ruger but Browning and Weatherby for sure. There is a very good post by a former Browning smith that spent a LOT of time fixing salt wood guns. (Try a search)
So not just salt wood but a huge increase in total production which probably made QC a much more iffy proposition.
If I was looking for one with the highest likelihood of good QC AND a Douglas barrel I would keep it under 130-5000. You would also get a rounded edge 1st generation butt pad, no barrel warning, a three screw adjustable trigger, first generation checkering and serial numbered breach blocks. As a semi collector, I'd say that once you get to 1980 the only true rarity is in the cartridge it's chambered in, as the engineering and cosmetic changes have all been done and the rifles are essentially unchanged since then with a few exceptions. (1984 change to the front angle of the front of the receiver)
All of that said one of the most accurate #1s I own started life as a Wilson barreled 218 Bee "S" that was rechambered to 22BR. The stock was so plain I sanded it and painted it blue. ;-)
There are still the #1 that shows up with killer wood but if fit and finish is your goal the chances are much higher with an early gun. They simply had more TIME and a different attitude regarding the #1 in that time frame.
Ross
PS: You NEED the book. ;-)