Exactly right BP. They were pretty common in the desert Southwest. I believe they were originally a Spanish design usually used for wine; made from leather. I'm sure the Spanish brought them to the New World and now Mexico has continued the tradition.Bota Bag. Used to carry wine on hiking trips in them.
I have a Stanley hip flask, but a much newer version. They are more upright, and not curved to fit a hip pocket. Still works.I have this Stanley hip flask but mine is green just like the thermoses. I have two of those. One bigger one and a wide mouth one for soups. I bought them from sports stores They are classics.