https://stampselector.blogspot.com/2010/02/stamp-investment-tip-mexico-1934-35-20c.html
StampSelector: Stamp Investment Tip: Mexico 1934-35 20c Slate Color Error (Scott #C62a)
In 1934-35, Mexico issued a rather unremarkable set of three stamps reprising the Coat of Arms/Biplane design of 1929-34 (Scott #C62-64)....