Not much to add. Your gun is a W. Greener. Most of the time when folks talk about owning a "Greener" they are refering to WW Greener, his son, a Birmingham maker of some note. Your "antique gun guy" must not be in to English doubles.

I have no idea what the value would be. You will see a ton of WW Greeners on the second hand market, but no so many muzzle loading W Greeners.