Now, here is a rather common type of British holster, used for the Webley MkVI revolver and the American made Smith & Wesson and Colt revolvers that were made in the United States for the brits. All in .455.
What is not so common is where the holster was used. It belonged to L. P. Sedgwick who joined the Royal Flying Corps in 1916. He flew through the war and survived. So, this holster has been in the air in WWI.
The revolver is a Smith & Wesson in .455. It is the second version, known as the double lock, as opposed to the more technically complicated triple lock, which was discontinued and today is quite rare. So, this one is more common, but a pretty nice example.