Check on the barrel, under the telescope, for additional marks. Often, a maker would scratch his name there.
Is the false muzzle still with the rifle? If so, that would be a great find as they were frequently misplaced after the original owner lost interest in shooting or the piece was sold to a shooter not interested in target work.
Interesting piece, the trigger appears to be a settable variety.
How wide is the barrel? Any guess as to caliber? Some of the old target guns were quite large, both caliber and barrel.
By the way, not the first inline, I have read of a flintlock inline from Britain in several museums.