First of all, which models were the same for both markets? Identical models include The Monsterbots, the Decepticon Targetmasters, the front of the Horrorcons, and (oddly) Chromedome's car. Modified models include Mindwipe and Weirdwolf. (Note: The Ark 1 includes the Japanese models for the Decepticon Headmasters instead of the proper American ones. I didn't have the American models at the time. This will be rectified in the upcoming Complete Ark Omnibus, but more on that later.)I have signed model sheets for the backs of the Horrorcons from an artist named ‘Miki’, who had also drawn several other models who’s Japanese design differed from the American ones. This is an important clue. It is odd to have a different artist do the front from the back, and we know Miki was a Japanese artist. The most likely explanation is that the Japanese studio received the American models for Apeface and Snapdragon but needed backs for them, and so produced them in-house.
Another clue is the models for Mindwipe and Weirdwolf. They are similar in both markets, but The Rebirth models have extra elements. Especially in the days before easy digital manipulation of images, this suggests that the American market modified the Magami Ban models produced for the Japanese markets.
The final piece of evidence is which models were the same in both markets. Note that most of the models that are the same in both markets came out in Headmasters AFTER The Rebirth aired. The Horrorcons debuted in the December 1987 episode Daniel Faces His Biggest Crisis Ever!! The Decepticon Targetmasters debuted in the February 1988 episode Miraculous Warriors, Targetmasters. Of course, the exception to this is the Monsterbots, who debuted in the July episode Rebellion on Planet Beast. Since the Monsterbots weren't in The Rebirth, though, it's unknown when their models were created. I don't have information about when their commercial first aired.
This in turn gets to the heart of the matter: timing. The Rebirth aired in November of 1987. Four Warriors Come Out Of The Sky, the first Headmasters episode, aired in July 1987. We know that the models for S1-3 were produced in America. It seems probable that The Rebirth models simply weren't far enough along to be used by the Japanese studios for early characters, so they made their own designs. This was the first bona fide new Transformers cartoon produced in Japan, and they certainly needed modelers, if only for the backgrounds and Japanese exclusive characters. (As to why Chromedome's car mode (and only his car mode) was the same in both markets, we can only speculate.)
As to why the American studio didn't just use the existent Japanese models, who can say? Perhaps it was because they already had modelers on staff, or perhaps it was due to aesthetic issues. The Japanese choosing not to import other later characters who would presumably be available, like Punch or the Clones, might also be chalked up to taste.
So, that’s why there are two different sets of models. Two different studios with deadlines four months apart conspired to produce two different sets of models, with some coordination later in the game explaining what overlap there is.
