This is a British Army open top holster from the WW1 era, used to hold the Webley Mark VI revolver. They were issued to enlisted troops such as machine gunners, tank crews etc.
There are no visible markings that I can find on the holster. Leather is still supple and stitching is good. Works as designed with two buckles for belt adjustment, stitched sleeve for cleaning rod and leather strap with brass button to close over revolver once in holster. Good overall condition for an item over 100 years old!