Never heard anything about the alloy of the recievers. But what was commonly known was that unlike the changes made to the model 70 rifle, there were no mechanical changes to the model 94. The changes were in the metal forming process used for certain parts.
Specifically, the lifter/carrier was changed to a stamped, rather than a forged part, and there were a few other changes to cut production costs.
According to urban legends, the stamped parts "weren't as good", and Winchester did have some QC problems with the guns made in the first couple years after the change.
I can't speak to this directly, but it was what "everybody" was saying in those days.
Now, while the Model 70 was actually a different rifle after 1964, the model 94 was not. However, the idea that pre-64s should command premium prices bled over (in a lot of minds) from the model 70 to the 94. I never felt that way, about the 94. A 1951 model 94 in good shape is an honest gun, as good as they ever were. Now what you have to pay for one in todays market? That may not be what I consider an "honest" price.