When I saw Pres. Luis Echeverria Campins's bodyguards near my Cancun hotel in 1980, they were armed with CAR-15's and M-16's for the most part, and wearing black fatigues.
Special unit. When I saw the kid, wearing standard green Mexican army fatigues, at the "agricultural checkpoint", between Yucatan and Campeche states (on the road from Merida to Campeche) he was armed with a G-3. Kid was behind a brick wall of some sort. The fat old guy did the inspection, the kid provided overwatch, just in case the Suburban with a mixed batch of Gringos and Campechanos was smuggling bacon into Yucatan. :)