The proofmarks on my J P Clabrough & Bros sidelock are from the 1887-1904 era. This is also past the 1883 date you found. I believe that first Clabrough & Golcher & then Clabrough & Johnstone were companies set up in California, USA for distribution purposes to an Amerian market. Apparently many of the guns themselves continued to be marked with the original makers name ie J P Clabrough & Bros.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra