I can't see them very well but probably Birmingham private proofs. If so, they pre-date the Birmingham proof house marks. TK could be Thomas Ketland. If so, I suspect it was just a way to identify whose barrel it was. The evidence we now have indicates that the private proof house in B'ham was owned by the Galton's and that they proved barrels for many makers. TK had a son-in-law that was a barrel maker.