I once owned an absolutely mint Smith that looked the same on its underside, right behind the frame. I never figured out with certainty what caused it since the gun was probably never out in the field nor even out of a gun cabinet. I finally chalked it up to some sort of chemical reaction that caused the varnish to "bubble" behind the frame, probably where a solvent or whatever likely soaked into the inletting cuts there.
The bubbled varnish on the Smith looked just like a kitchen table we once owned. The table was a high end one from Ethan Allen and while it's surcase was walnut-looking formica, the end pieces were real wood, varnished or maybe lacquered. After many years the finish bubbled up and the wife always said it was because she used a liquid furniture polish on those end pieces. Silvers
