I once had a root beer mug collection and I never heard of or saw one of these. I suspect it is likely worth a couple hundred or more, but priceless if I dug it up, as it would find a spot of honor in my collection of oddities. The shape & style remind me of the earlier Villeroy & Boch Mettlach from Germany mugs that made the earlier Hires mugs. Also that rim is also indicative of an earlier style, say late 1880's. SUPER nice find! Jack
I hadn't yet bothered to sign up since the forum change had taken place, but after seeing your mug I got it done just so I could say... that is SICK!!! Top shelf find!