Very interesting Jerry. I tried to enhance the contrast of your image without much success, but there is definitely a crolle pattern with smallish scrolls; it could be 6 Iron. Could you post an image of a section of barrel with more contrast?
This is a 1878 Grade 2 with a 'D' and difficult to see large scrolls; likely 3 Iron "Oxford"
Another Grade 2 with 3 Iron
A 1874 $250 grade with 3 Iron "Oxford"
Another 1874 $250 grade with 4 Iron "Turkish"
$250 grade with 6 Iron "Turkish"
Unfortunately, I don't know if the flats had nothing, 'D', or 'DD'.
For what it's worth (not much) I believe the D3, D4, B, and DD or D6 marks started with the hammerless guns.
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