That's a British one size fits all holster for the 38 Enfield & Webley, Victory, 1911 and BHP.

The Blue/Grey color IIRC was for RAF issue and holsters were usually (not always) marked AM (Air Ministry) and dated.
Manufacturers markings are also usually inside the flap. Sometimes inventory numbers, etc put on in the field.

Markings can be hard to read.
WW2 mfg has the (P37) Brit belt attachment brass hooks on the back only.
Post WW2 mfg holsters look identicle except they have a brass hooks belt hanger on the back allowing not only attachment to the Brit issue belt, but the US GI web belt as well.

You used to be able to buy the matching canvas belt and (revolver)ammo pouch that went with them quite inexpensively also. Or the entire set.
Possibly you still can.

Some leather holsters were issue. I had and sold not to long ago an Australian property marked&dated 1941 flap holster for the Victory. It had an english makers stamp on it also.