Mark found on a pair of knee buckles with the London 1756 — 1784 Lion Passant .
Perhaps an unrecorded mark of John Deane after leaving the partnership with William Deane c1768. Both Deanes are listed separately in the PR1773, both without a category.
Grimwade gives John as registering a JD mark without pellet in 1768 (No1797). There are very few marks starting with the “J” before 1770, the “I” being preferred by most James’s John’s and Jeremiah ‘s until later.
Although the original partnership mark of John and William Deane (Grimwade 520 et alia) is seen on buckles from time to time, I’ve only seen one JD on a buckle . Presumably John did other things after his split with William, as you expect to see more post 1768 buckles than earlier one .