Here is an example of an undocumented change in stainless steel case manufacturing after Bulova presumably discovered their watches were corroding. The model in question is the Ambassador with the solid gold bezel.
The Ambassador line of watch in the 1960s have stainless steel backs, however, most of the cases I've seen are badly pitted. The examples I've seen are dated from 1966-1968. Coincidentally, these cases are all made in W. Germany. I guess their stainless steel metallurgy wasn't quite up to the abuse of natural oils of the wrist.
I deliberately sought the same watch model (Ambassador) with a stainless steel case made in Switzerland. Mine is dated M9 (1969). The quality of this stainless steel is what you would expect; there is no corrosion. There is one tiny change to the dial though. Instead of the word "Selfwinding" the phrase is "Automatic".
Here is a photo of a pitted case, stamped "W.Germany" inside: