Frank looked like, acted like, and sounded like, Biff Tannen. It's likely that he is Biff's uncle, as well as the brother of Kid Tannen and the son of Gertrude Tannen (who, by 1955, was "the only Tannen in the book").
Sometime after 1944, Frank no longer lived in Hill Valley and by 1955, Biff lived with his grandmother. Biff stated that he had an uncle who abandoned him, so it's likely that Frank left Hill Valley at some point before 1955.
Behind the scenes
- In the episode, Frank uses German expressions twice, calling Marty a "weisenheimer" and a "dummkopf", and corrects himself both times. However, his conduct as a sergeant in the United States Army does not appear to show allegiance to any other country.