They stopped production in the 60's. If you're from the Wisconsin area, you may find them in antique stores, or get lucky at a garage sale.
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I am by no means an expert, all though I do own my share of cast iron cookware! Castironcollector.com has lots of resources and you might be able to have a member help you date your skillet. It is my assumption that there was a wide period of time where they stamped their name and an even larger period of time where the heat ring was cast. From what I'm able to gather, it is less frequent that you find a Vollrath without a heat ring.