This film was originally released in the U.S. in a 2-D version. That version of it was pulled during its theatrical run (despite the fact that it was doing good business) and replaced by a 3-D version that failed at the box office.
The film's original intention was to set its story in Spain and for it to be about a Galician or Asturian werewolf, but the Spanish censorship of the era would not allow such a story to be set in Spain or such a character to be a Spaniard, so it was set in a Teutonic country and a Polish werewolf was created.
Frankenstein does not appear even once in the entire film in any way, shape or form.
When Samuel M. Sherman was screening foreign film titles for a suitable Frankenstein film to release to distributors for a promised Frankenstein double feature, he actually passed on the film "The Monsters of Terror" (1970; which was originally going to be titled "The Man Who Came from Ummo" before getting the title it now has), Paul Naschy's first of 11 sequels to "La Marca del Hombre Lobo" which was completed in 1967, two years before this sequel was in 1969. The film Sherman had passed on combined Naschy's tragic werewolf character Count Waldemar Daninsky with the Frankenstein Monster, Count Dracula and the Mummy (all three of them, however, appeared in the film under different names) and starred Michael Rennie (this was Rennie's final film before his death) and Karin Dor, two internationally-known actors.
This film was Jacinto Molina's debut as a screenwriter. He was also convinced to star in the film as well, thanks to the German backers who salvaged the production. Forced to choose a non-Spanish acting name for the international market, he picked "Naschy" from a friend, Imre Nagy, while "Paul" was inspired by Pope Paul VI.