@NewChrissy That's a good question, I have never known if it was a single factory where they did bootlegs in the 70s in Japan or it was a couple of workshops. But as you rightly say they apparently copied the heads of the Bullmark figures and used them generically in various figures of Japanese bootleg heroes. For example, in the case of bootleg toys made in Mexico in the 80s, they were made in various workshops near Mexico City and from there they were sent to different parts of the rest of the country, but apparently it was the same person who made the sculptures of the toys that were going to be produced or the molds that were going to be used in other workshops, I don't know if something similar happened in Japan because the production process was more traditional in both countries. Something has these bootleg figures that always catch my attention, especially those with removable masks.
よい子の おもちゃ (Good Child Toy) I believe that Brian Flynn has one of these with a "Kaiju Set" header, so further proof that the toys are just bagged up and stapled with whatever random header is printed up at the time: (photo from Super 7 Isssue 1)