1964
In 1964 Winchester made a major change in their product line and manufacturing methods. This was done as a cost saving method, to make their guns cheaper for them to build.
Many people didn't like the changes made in several models, and sales (and reputation) suffered for some time.
Basically, in lever guns and shotguns, many of the minor parts were changed from milled (expensive to make) to stamped (cheaper). In some cases the early stamped parts were not entirely satisfactory.
In the case of the Model 70 bolt action rifle, it underwent a major redesign. Winchester redesigned the bolt, dropping the large Mauser style extractor, in favor of a smaller one. They also free floated the barrel, creating an "ugly gap" around the barrel, where previous rifles had the wood closely fitted to the barrel. There were other changes, but those are the most obvious ones.
Initially the smaller extractor was not trusted to be positive, and manufacturers of dangerous game rifles would use only "pre-64" actions. Because of this, and a general feeling that Winchester had "butchered" (cheapened) a fine rifle, pre-64 rifles came to demand a premium price.
This is just a thumbnail explanation, there are whole books on the subject, which go into all the detail you could want.
I have own a post-64 M70 in .22-250, and have no complaints.