I have no proof in any form and can cite zero, but streamlined manufacturing being what it is, and giving a nod to information above regarding S&W changing to all Magnum frames because it's only logical from a mass manufacturing standpoint...

I can see no reason whatsoever why there would be even a minute metallurgical difference in the construction of the cylinders between .38 and .357.

From all we have seen from S&W specifically, it simply wouldn't add up.