The 2nd Panamerican Postal Congress was held in Mexico City in 1926. The commemorative series contained a 1 Peso stamp that was very special.
For the first time in the world history of stamp printing, a BICOLOURED recess stamp had been printed from ONE single plate using an advanced way of inking. It took 12 years for next postage stamp to get issued in a similar manner printed by the PWPW in Warszawa in Poland.
In 1934 the sizes of the postage stamps were standardized by the Oficina de Hacienda - the 2 sizes received their own perforation blocks! Everything printed in screenless photogravure with an eventual exception for the higher values.