The label belongs to a shop. S Marco is a region on Venice itself, not on Murano. Here
is a map, which shows where the shop is located.
Hopefully someone can attribute your snail for you, or maybe you could even email the shop, as they should know where it came from originally. It isn't obvious, but if you look under the address, where it says additonal information, telephone, if you click on telephone, it automatically takes you to a page where you can send an email.
Hope this helps, a little?