Without markings all I can offer are some general observations:

1. Appears to be made for a revolver with relatively long barrel.
2. Definitely made as a swivel holster. The swivel mechanism has probably corroded, preventing movement.
3. Relatively broad through the cylinder/trigger guard area, which leads me toward a conclusion that it was made for a double-action revolver rather than the common single-action models.
4. Multiple holes punched through the flap for the retainer stud, which leads me toward a conclusion that the holster may have been used for several different revolvers over its lifetime, with holes punched to move the flap to better secure a revolver other than the one originally intended.
5. Overall design appears consistent with British Commonwealth military holsters of the pre-WW2 period. I would suspect Canadian or other British connection. Assuming the Commonwealth connection, I would consider possibilities for British revolver models such as the Webleys, Enfields, etc.
6. The small leather tab at the muzzle end on back of holster is probably intended for a strap that could be buckled around the user's thigh to prevent the holstered revolver from flapping around or causing discomfort during some activities (horseback use, etc).

Can't think of any more clues on this one.