Back in the mid-1950's the U.K. decided to sell of the remaining .455 and .38 revolvers they still had, and private citizen gun ownership laws being what they are (or were as the case may be) around the world, most of them ended up on this side of the Big Pond.
New Service Colts and Smith & Wesson Triple-Locks sold for $17.00 to $25,00 in very good to excelent condition, but chambered in .455 Eley. Many, if not most were rechambered to .45 (Long) Colt. Going to .45 ACP required a new model 1917 cylinder, but some were so changed.
Because of the exceeding high price
these revolvers went for, many of them became the basis for all kinds of custom conversions.
One problem with the "Brit connection" is that the frame appears to be from a model 1909 U.S. revolver, but some were sent to England during the early days of World War Two before we got involved. They however were all in .45 Long Colt. However the folks "over there" could have switched in a .455 cylinder. Trouble is, if all this happened I'd expect all kinds of proof marks and ownership stamps on the frame, cylinder, and barrel. Also I don't think they would have changed the cylinder latch thumbpiece, nor would they have fitted a 7 1/2" barrel and aftermarket adjustable rear sight. To further confuse the picture the barrel and crane have Colt inspector's stamps which make me think that the barrel is indeed original to the frame. If so, then it wasn't a U.S. Model 1909, but these were intermixed with commercial production.
OH DARN!!!!!