I'm not sure I understand your question completely, but will answer to my best ability. First of all, not all Grade O guns have Damascus barrels; as some later examples will be fitted with "London" fluid steel barrels. As to "Chain Damascus" barrels, they were optional on at least the Grade 2, 3, Pigeon, and 4 guns; and I've even seen a couple sets on Grade 5 guns. Chain pattern Damascus is very attractive and could be had in various grades that varied only in fineness of pattern and in the amount of steel content relative to iron (tubes with tighter/finer twists, and tubes with more steel content were more time consuming to make and thus more expensive). The chain pattern tubes Hunter used in their Grade 2 and up guns is quite striking and was a very good seller; and the terms "Damascus Steel" and "Chain Damascus" are, for the most part, generic terms to describe hand twisted tubes. The probable reasons for these generic descriptions is the fact that there are so many different types, grades, and patterns of Damascus and Chain Damascus tubes that only the people who made them could distinguish each type. In fact, in today's world, there are only a handful of really good barrel smiths/collectors who have the slightest clue as to how to ID/differentiate the myriad types. As to the Damascus descriptions Brophy re-capped in his book, those were terms taken directly from period catalogs. These phrases were designed and intended to sell guns, and varied with production periods (some may in fact refer to the same barrel quality and pattern). Hopefully our resident Damascus expert, Dr. Drew will post shortly and clarify everything.