Dave: One of the features of your thread is the amount of information that is not provided. Only now you mention that you have a sister who has at least some of your father's documents, and that your father died in 2001, i.e., recently. You are also calling your father "Dad" which likely means that you saw, and spoke with, your father many times, and likely grew up in his family. What happened to his documents after he passed away? Did he leave Austria, and if so, when? In which town and country did he marry and when? What happened to his mother? Where and when did she die? Was your father her only child? Did she raise your father, or did he grow up with foster parents?
As I already mentioned, there should be a Pflegschaftsakt about your father at the Bezirksgericht (district court) Linz, or at the Standesamt Linz, or at the State Archive of Upper Austria. As son of your father, you would have a right to a waiver from data privacy rules and get a copy of the act (=file). Among other important information, the act would clarify whether the father of Erich was identified.
By the way, when checking the Austrian on-line phone directory, you will not find many Benischeks and Kampmanns with a listed number. The addresses seem to point to Lower Austria and the region between St. Pölten and Amstetten.