1875 Parker Hammer Gun Question

[Scott Fox]

I have recently purchased a 1875 Parker hammer gun serial # 6359. The Parker research letter I obtained states that it is a 0 grade, 10-gauge, underlifter hammer gun, with 30" plain twist barrels. Now from my research in TPS, I believe that it was possibly entered incorrectly? I believe that it is in fact a grade 1. The barrels are marked Stub Twist, the stock and forend both have checkering, the locks, hammers, triggerguard, frame, butt plate are all modestly engraved. According to TPS this would be a grade 1, if I read it correctly. All numbers match on frame, barrels, forend, and triggerguard. I would appreciate any feedback that more knowledgeable Parker folks could provide.