I cannot remember having heard his real name until sometime in the 1990s. In the book, he's definetely only named "der Alte", same in the TV series. Same with Buchheims later book "Der Abschied". Does someone have the movie "Das Boot" and can check the cast in the credits? My impression is that someone found out the real name and placed it in the cast list. As "Das Boot" is not actually biographical (it was sold as a novel, after all, and Buchheim made substantial changes to events), there is no reason why this character should be called after the real person it was modelled after.