[Chuck Schwark]

Production Change Notes usually extended a couple years from the introduction of a model. Some models had extensive changes. The Model 60, for instance, has numerrous changes spanning a few years. Many models had none.

Improved engineering designs, avalability of components or new style components, value changes for improved reception/stability, new/better tube changes, etc. caused changes for the most part.

Ron may have more insight as well . . . . .