This mark has been seen on a cast pieced pair of tongs, and on a stock buckle. both with the post 1755 Lion Passant Guardant of London
Speculatively I suggest that it may be a lost smallworkers register mark of John and William Lutwyche .
I'm afraid the evidence in support of is a bit scant .
On 4 April 1764 the Freedom the Broderers Company by patrimony of a William Lutwyche is recorded . William is described as "son of John Lutwyche , late Citizen and Broderer of London, deceasedof , Goldsmith in Lombard Street "
A partnership between father and son would invariable show the father's name, as senior partner. We know nothing of John, but William Lutwyche is recorded as being involved in at least two thefts of silver buckles from his shop.