Good question: but the patents would have probably been on the basic system, not the cosmetic changes that made the MK. VI.
The gun used patents applied initially to the Webley-Kaufman and Webley-Green guns from the 1880's, and there may have been updates. The government revolvers from MK. I (1887) to the MK. IV (1899) improved the method of removing the cylinder and in the extraction system. The MK. V (1913) merely increased cylinder diameter over the MK. IV, to make it safer with smokeless powders.
The MK. VI used a butt shape that dates from at least the Webley-Wilkinson of 1905, which closely resembled the MK. VI, except that it was better finished, and lacked the triangular holster guides in front of the cylinder. (The Wilkinson series was updated in 1911 with a stronger cylinder.)
I suppose it is possible that the patents had run out, but the government didn't have to worry about that, as it couldn't be sued. Moot point.
If anyone does know the limit of the Webley patents, please post. I'll try to find that info, also.
Webley sold the MK. VI to the government from 1915-1918, maybe later. It was still available from Webley commercially until at least the opening of WW II, and was shown in Stoeger's catalog as late as 1939. I once saw the ad in an old "Shooter's Bible".
Lone Star