I wouldn't feel the least bit handicapped with one of the XTR's made from about 1980 until 1992.
I will echo that sentiment....really nice rifles...
My research supports the climb back to higher quality as you move forward from 1964 and the A and B models.
I tend to believe that the real craftsmen (there are always some) among the staff preferred to create a high quality product.
There are writings which indicate that there was a "bitter" strike from sometime in 1979 to some point (perhaps December) of 1980. The board made the decision to sell the plant in December 1980. The strike was reportedly a Machinists strike. Without knowing someone who was there at the time it is hard to judge the effect on quality, however production volumes were way down in '79 and '80.
I have seen no information as to whether production continued with parts in inventory, or if some other solution was found. They did, however continue to build model 70 rifles.
Use this information as you will in valuing a rifle that you are considering.
Typically lower production allows for better QC, but disgruntled workers, worker shortages, or substitute workers may counteract this tendency.