You have to be real careful with snail drums. They are worth a good chunk of change (starting at around $1500 with condition issues and going up to over $2K) and, as such, there are several fakes floating around. Obviously the pictures that are on the ad leave a lot to be desired for authenticating the stock and snail drum.
The consensus on the leather snail drum holster is they are all fake. They started appearing in the 1950's and are usually marked with the date "1915." The problem with the 1915 date is that the Snail Drum magazine wasn't introduced until 1917, which begs the question, "Why produce a holster for something that wasn't even produced for another 2 years."