As Dean stated, D with a number close to it is the number of crolles used in making it. I had a grade 1 hammergun with D2 bbls. I've had grade 2 guns both hammer and hammerless with D3, D4 and just D. The grade of gun stamped on the barrel flat started with Grade3 guns as Dean previously posted and is stamped closer to the breech end of the barrels on the right flat.
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