The proof marks date the gun to 1875 to 1887 and I doubt that you will get much closer than that, all records being lost to the best of my knowledge. The patent is no 651 of 1876 so that removes another year. My Crockart example in stock is numbered 391, which could easily be a Woodwards seriel number, and carries the same 1875-1887 proof marks so arguably yours could be from earlier than latter in the proof span. You can view it on http://www.heritageguns.co.uk/Crockart%20SLNE/Crockart%20SLNE%20Details.htm.