These stamps can be spotted easily on cover because of the high contrast of the bright/highly reflective white paper they were printed on.
Here are two selections from covers in the Continental Grain Correspondence,
Continental Grain Correspondence 1 Peso covers page
and a superb find: the extremely rare 2 pesos clay from 1943. Never mind that it is on a 5 pesos cover-that alone would make this cover rare, but the 2pCL1B is the find here, and not the 5 pesos, which is most likely a 5p1E4.
The very rare 2 pesos on cover (quite a find!!!)