Angus,
FYI, the SCT was the radio licensing authority back in 2008. In my meeting with the Jeffe at the SCT office in Zapopan, where they have an all band radio monitoring station, I asked him if he had the authority to grant me a Mexican licence and XE1 call sign. He said he had the authority to grant me reciprocal (to the FCC Extra Class license) operating privileges while I was a FM 3 visa holder. I asked him for a letter of authorization to operate, and he said his verbal authorization was sufficient. I wanted him to issue me an XE1 call sign, thats when he told me to use my FCC call sign portable XE. A ham from Guad, XE1RM, was with me at the meeting and confirmed that the SCT official had the authority. Over a ten year period, I tried to get the Mexican government to issue me a paper license. No va! I finally took my radio antennas down and sold off my gear. BTW, I have been continually licensed since 1961 and hold the Amateur Extra Class and the commercial First Class license. I have also been licensed to operate and held call signs in Nigeria,Samoa, Tonga and Belize.
73s