The back looks like it used to have some sort of covering piece going across the back, which is now mostly gone - there's still a remnant of it on the upper left corner as shown in the photo. The metal frame has a few layers of paint, with the top layer being an army greenish color that's crackling and has great patina. There are small metal pieces on the upper edges of the frame on the sides - I don't know what they're for - I assume they're part of the way the sign was mounted at some point. I'm guessing it's from the early 1900's but I don't know for sure.
It's a rare old piece with great subject matter. The item "RaRe Vintage Antique BOAR'S HEAD TOBACCO Reverse Painted Glass Sign" is in sale since Saturday, September 9, 2017. This item is in the category "Collectibles\Advertising\Merchandise & Memorabilia\Signs\Original\1930-69".
The seller is "funkytownantiques" and is located in Marietta, Georgia. This item can be shipped to United States.