King's Parent that included the recessed hinge-pin feature had run its course when Ansley H. Fox used it on his Fox Gun Co., Balto., MD., U.S.A. guns in 1898 and 99 --
and the Philadelphia Arms Co. Fox doubles 1902-06 --
The recessed hinge-pin Sterlingworths were 1910 --
to 1913 --
Some of those early Lefever hammer guns with the recessed hinge-pins might have infringed a bit!! Or, maybe they are where King got the idea!!