It's not a Mk1, but it sure looks like a Webley flare pistol to me!
I jumped off my couch when I saw this.
From season 2.5
Episode 1 "Pegasus" extended DVD version
Not just a revolver, it looks like a version of a Colt carbine with a removable shoulder stock, very nice piece of history. That whole collection looks like it should be in a museum.
Alan
P.S. Found this pic of the shoulder stock option. Maybe not a direct match, but very close.