If you look at the trivia for this movie, you will see that it was intended to be a direct sequel to "Frankenstein Conquers the World".
Much that made it a sequel was toned down or written out of the American script. The only actual referral being the conversation regarding the desicated hand. They do call them Frankensteins in the Japanese version.
9 years ago
Once Nick Adams wasn't available that's when they decided not to make it a direct sequel but the loose sequel frankly comes off as confusing and one of the things better about the US version is the elimination of the "Frankenstein" reference and just suggesting that the Gargantua is a totally different breed that had been raised in the lab.