This early Colt Double Action Army Model of 1878 was made in March 1879 and exported to London. It is London proofed on both frame and cylinder and has the Pall Mall agency address on the barrel. This example is chambered in the British .450 Boxer (.450CF) cartridge which was the standard British military revolver cartridge at the time. It also has the oversized rosewood grips that were a feature of these early guns.
The British War Department made an emergency purchase of 165 of these revolvers in March 1879 in response to the outbreak of the Anglo-Zulu war and the British defeat at Islandlwana. Unfortunately this revolver is not one of them (But I do have one that is pictured elsewhere on this website).
All in all a nice London retailed Colt in decent condition.