The only replicas of the Whitneys were made by the Palmetto company. If it's a repro, there should be Italian proofmarks on it, and a company logo (I think it's a palm tree).
What is the serial number?
The markings on your barrel top are the correct ones for an original.
The original frames were made of iron - mine has a plumb brown patina to it.
Remove the trigger guard and look for the Whitney Eagle or other marks.
I shoot .380 roundballs out of mine with about 15 grains of black powder. I would have it checked before firing it if you don't know what to look for.