It bears the stamps Mk VI on the top strap, and was made at least in 1917 from the patent stamp. It was made for cordite or nitro loads, not black powder.
It fired a 265 gr fmj bullet, rather than a lead bullet, in order to accord with the Hague Convention. This bullet was used during the Great War and the Second World War. The propellant was 5.5 gr of an undisclosed type nitro-cellulose.
Here in UK we used to shoot them with a 255gr lead bullet over 3.5gr of Nobel Revolver #2 - about the same as Unique or Bullseye. Velocity with this load was just under 700 fps, my notes say.
Experimentation is recommended, starting with about 3gr of unique or Bullseye and watching for pressure signs.
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Velocity was about
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