I have Koppenhoefer's in my family tree dating back to about 1705 (Johann Jakob) in Frankenstein, including your grandfather. I immigrated to the US from the neighboring village of Weidenthal (my ancestors are from Frankenstein also), and have been involved in genealogy for +25 years. I have an extensive family tree (+38,000 and counting) covering the general area of Frankenstein, as well as an extensive library of village history books and family books. Unfortunately Frankenstein doesn't have a history book yet, but the district seat in the neighboring village of Hochspeyer does.
Although I'm not directly related to the Koppenhoefer's, I have at least 3 Burckhardt relatives from Frankenstein that married Koppenhoefer's. As I'm a bit nuts when it comes to genealogy, I'm always interested in adding relations to my family tree. I'd be happy to exchange information with you. Having found your message, I found it very interesting that the spelling changed to Koppenhafer in the US (in German, Koppenhafer doesn't sound anything like Koppenhoefer). I am also directly related to the Nabinger family (saw your message on that family also)