The sequel to this film is The War of the Gargantuas (titled Furankenshutain no Kaijû: Sanda tai Gaira in Japan). In said film, pieces of Frankenstein's cells mutate into two giant humanoid monsters: Sanda (the Brown Gargantua) and Gaira (the Green Gargantua). The former is a benevolent and peace-loving creature, the latter is murderous and savage. Apart from a reference to a severed hand, the US co-producers ( United Productions of America ), obscured all references to Frankenstein in the American version, possibly because the two monsters could not be recognized as "Frankenstein" monsters. Gaira and Sanda would later appear in two of Toho's Tokusatsu series, Ike! Godman and Ike! Greenman , before remaining absent for over 40 years, with Gaira making his latest appearance in a 2008 Ike! Greenman special.
There are apparently several video versions of the Dracula Vs. Frankenstein film you reviewed from 1969 (more appropriately named, Assignment Terror), even though several of those versions use the exact same cover you showed. My version is better in that you can see most of the film, but you have to pay attention to the run times. I bought mine in the early 2000s and it runs 77 minutes. Newer versions run as little as 61 minutes, and I’ve seen versions that run 71 minutes as well. You can download a version at Amazon that seems to run the full length of 87 minutes, and this should clear up some of the editing mistakes (some of which are cut in mid-sentence).
And you never mentioned that this film was a Paul Naschy (real name Jacinto Molina), a Spanish horror LEGEND, who wrote and acted in many films and made something like 14 werewolf films. So really, this is a werewolf film, not a Dracula or a Frankenstein film. You also mentioned that Dracula has a different name, but so does the Frankenstein monster (it’s the Franksollen Monster).
I’m a Paul Naschy fan, and I hate what the video distributors did to this. In its original form it was a cheap, but fun action/horror movie, not the incoherent nonsense released under this title (and it was never even called Dracula Vs. Frankenstein until it was released on VHS over 20 years after it’s original release!!!) I suppose this isn’t really your fault, but when I was kid watching this on a late night hosted horror movie show, I fell in love with this film, and it was nothing like any of these VHS versions floating around. They are practically two different films. You could see the action, and people finished their sentences before it faded to the next scene. I’m sure the complete version will not be Citizen Kane or anything but the marketing and distribution of Assignment Terror has been a travesty.
Anyway, another trivia note, the female lead is played by Karin Dor what was not only a Bond-girl, but also appeared in Hitchcock’s Topaz.