We have a Hot Water Bag Rubber Miniature for Dolls, Vintage.  It is brown and measures about 2 1/8" in L x 1 1/8" in W x about 1/2" across the open spout.  It is marked "Dolly"in the center on one side.  I'm not certain of the vintage of this item but it was acquired with other miniatures which dated between the 1930's - 1970's.

Hot Water Bag Miniature for Doll Vintage