Very interesting and great research work! From what I understand, I agree too with Brazil being the most fitting origin, even if it's one of those cases where it's particularly difficult to determine, but I (personally) think that the main design team being brazilian "wins" on the other involvements.
About the code, maybe [GMT31XX] is a "retroactive" way of referring to the whole platform project, whereas [GMT700] was the code for the single product (in this case the Colorado)?