While I freely admit that South American history is not my strong point, a bit of Googling around yielded some interesting results. The figure labelled "Napolean III" is not a bad
likeness. (I agree that the sans serif font of the stamp appears modern, but the letters appear to be well aligned, not individually stamped as is often the case with post hoc "embellishments"). The other male figure bears more than a passing resemblance to
Orelie-Antoine de Tounens, a French adventurer who styled himself "King of Auraucania and Patagonia" during the same period when Napoleon III was supporting the Emperor Maximillian in Mexico.
The figures could be simply generic military men as you suggest, but there is at least some resemblance to actual historical figures relating to a common theme of French intervention in 19th Century New World politics.
Edited to add:Further research indicates that sans serif or grotesque type was in use at a much earlier date than I realized. Note its
use in a French publication c. 1898. And the
Phrygian cap worn on Liberty's head is associated with both the French Revolution and the Argentinian coat of arms (among others).