Hi!
By the 1950's the NW guns had been replaced by the Imperial Shotgun. There is a NW Gun with the lockplate dated 1900. That's the latest I'm aware of. The "19" and the "00" are done with different dies which may indicate they were considering going forward. 1900, 1901, 1902 etc.
I have a letter from York Factory (fur post) to HBC headquarters in which the Chief Factor (boss) wants to send back flint guns to convert or be replaced by percussion guns "as they are no longer of any value here". I believe the date is 1887. So we know that by 1887 even as far out in the bush, York Factory was, a supply of percussion caps were no longer a concern.
I have a flint Barnett which is dated 1876. I've been fortunate over the years to have found 14 original NW Guns. Earliest a 1821 gun ordered by the NW Co. but taken delivery by HBC.
Hope the above has been of some interest.