I suppose I should add a song fact with the image.
The only one I know, and I heard this from a DJ on a Philly radio station, is that this instrumental to so long to record, and went through so many different phases, takes and retakes, that somewhere during the process, someone in the studio said they had created a monster.
From then on, they referred to it as Frankenstein.
How factual that is, I couldn't say. I am just parroting what I heard a DJ say.
I will say this about the song: it's the earliest recording I know of with 'hammer ons' fret work. For those of you not familiar with it, it is the sound Eddie Van Halen gets during 'Eruption'. A lot of people think he was the first, but Frankenstein has it.