1 Onza Mineral Resources Convention

Struck to commemorate the Fourth International Mining Congress held in Mexico City in 1951, this piece was produced in very limited numbers for distribution to congress delegates and officials rather than for general sale. It occupies an awkward category — neither a true circulation issue nor a standard commemorative — which likely explains why Grove treats it as a variety rather than a primary type. The distinction between 587a and the parent type concerns subtle differences in the lettering spacing around the legend, a detail that has generated considerable disagreement among specialists.