Based on the A&D use number, I would say 1887-1888, but the proof mark would narrow it down to 1887.
Those patents were good for 14 years, so the A&D patent expired in 1889, even though guns were finished sometimes later later with a Patent use number.
Both Anson & Deeley worked for WR. Anson was the foreman and Deeley was the managing director at the time. I don't think that they would have licensed the patent to others without the firm's approval and/or benefit. Deeley owned most of the WR stock in 1899.
The 1864 patent C bolt lockup is special to Westley Richards and would point to their direction.
Even though other brands have used a C bolt (Bland and many others), I would think that they would have sourced their action from WR.
Enjoy the gun!
WC-
