I believe it was looked up wrong. No doubt a British service revolver #4054 in the British serial # range was produced, however, never with a 7 1/2" barrel, custom Bisley sights and checkered trigger.
First there was a #4054 TL Military model revolver produced in the 44 spl # range but factory chambered in .455. The # (most #s) was duplicated but in the British contract serial # range about a decade later.
No offense to Don Mundell who assists Roy Jinks with historical letters, but I believe it was looked up incorrectly in the shipping records. I would call Roy and have it looked up in the .44 Spl Military 'Triple lock' serial # range, and once he confirms it, request a corrected letter.
Call Smith & Wesson @ 800-331-0852 and choose option 7 on the menu when the 'machine' picks up. The Historian, Roy Jinks, will come on the line if he's not out of town. Give him the serial number, refer him to your letter and the details of your gun, and he can look up the # in the .44 Spl Military shipping records from the factory. That will confirm the correct identity and configuration of your very unique gun.
Also refer Roy to post #28 in this thread. Roy knows Handejector and that he knows as much about these as anyone. Also as he posted, the lanyard ring in the butt is not a S&W lanyard ring like all British service revolvers had. Drilling thru the butt serial # ws not done on the BSRs, but could be a factory installation however only if it has the serial # also stamped on the left side of the grip frame near the bottom. If not, the drilling was not done at the factory. The BSRs had the serial on the butt offset so the # would not be drilled thru.
Let us know what you find out.