Herbert,
That looks like an unmolested G Grade! I won't speak to monetary value but you should treasure it as a rare piece of art from days gone by.
On the same size frame barrels of a larger bore and same length should weight slightly less than barrels with smaller bores. The barrel breach/chamber areas contain more metal on smaller bores because the chamber areas will be thicker on smaller bores.
The only viable method to determine barrel safety is to measure the barrel thickness from muzzle to breach in all available areas. Obviously the chamber areas need to be thicker than the muzzle. If in doubt take it to an experianced double gunsmith for inspection.
Mark
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