White Porcelain Shell Sign

I would like to know the history on this White Porcelain Shell Sign that I recently found. I believe it came from a Shell Gas Station, but not sure of that. Can someone tell me the history behind such a sign, the age of it, the use, the value of a sign like this. It is aprox 36"tall 36"wide 10"deep at the widest point. It's heavy and well made, has a bracket across the back for hanging. It is a very cool sign. I found it I'm Mississippi, the lady said she bought it at a yard sale. I would like any and all info I can get on it. I really like this sign! Thanks in advance.