I am afraid I have to differ from Martin for once. I believe the case of No. 7313 is American, like that of Keith's 4808, with imitation English hallmarks. As Martin says, the crowned leopard's head (the mark nearest the winding-hole) is not usually found after 1822, but it persists on the American cases mentioned by Keith. British casemakers did not usually decorate the bow, and the hunter style, while certainly not unknown in Britain, was far less popular than it was in the United States. The case is probably gold throughout, though possibly not quite up to the then usual 18-carat standard. The house of Johnson exported much of of its output across the Atlantic, usually in the shape of uncased movements; they attracted less import duty this way, and the jewellers of New York were perfectly capable of casing them worthily.
The original Joseph Johnson (1780-1827) had a son, John Houghton Johnson. He lived only to the age of 28 (1808-1836), but this watch may well date from his lifetime (middle to late 1820s, I would guess), and so there is no reason why he should not be the John Johnson named on the movement. At any rate, there seems to have been no other John in the family. His name may have been used to denote watches of a particular quality (a foreshadowing perhaps of the 'grade' system in the later American industry), or perhaps to indicate that they were sold through a particular outlet; we do not yet know.
John Meyer (mentioned above) has recently been posting as 'Hilbre' on this board. Coincidentally, I have been in touch with him this week and he has put me in the way of much helpful information which I shall pass on as soon as I have fully digested it.
Oliver Mundy.