Tony,
Welcome to Meximail. I looked up in the old Mexicana to see if the stamps you are interested in finding the designer of were listed in New issues. Well they have them, but the details they gave for the main issue, Scott C299, you are interested in is not detailed at all. Most of the other issues only list thing like perforations and watermarks. The Scott catalogue does as much. I did look closely at a reference stamp you gave, Scott C279, and the signature is better but still lacking any real detail. So, I suggest that maybe someone from Mexico on the forum may be able to find the answer in an older Mexican philatelic publication.
I hope you dive into Mexico deeper and join MEPSI again. There is a lot more we are finding out about Mexican stamps in this digital world we live in now. But, for now, it is great to have you on Meximail.