This is what I've read and came to understand. Whether the designation on these hats was a marketing ploy or not, I doubt we'll ever know. From the Leon Drexler site:
The softest fur comes from the beaver’s belly. Felt made exclusively from this fur came to be known as “silverbelly” due to its naturally light grey colour (which should not be confused with the word silverbelly as sometimes used to describe a similar colour of felt, but not the actual composition of the material). There is some modern-day hat aficionado mythology built up around silverbelly and its legendary softness and suppleness. However, one old text I’ve read states that felt makers actually considered pure silverbelly felt to be too soft, lacking somewhat in structural properties, and that a better felt is actually produced if you blend in some fibre from other parts of the animal to give it a bit more sturdiness. Tactile appeal aside, I know of no evidence that suggests there are any qualitative advantages inherent in silverbelly, nor in blending beaver with other luxury furs such as mink or chinchilla.
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