"Sargento Mayor Joanes CU DE ROYARN FDO"
(Disclaimer - Below is only what I have learned from various Hispanic friends, so take it with a grain of salt as I don't know if it's 100% correct)
Full Hispanic names are sometimes very descriptive, to include maiden names of Mothers, Grandmothers and first names of Fathers and Grandfathers.
Also, long names like "Fernando" or "Francisco" are often abbreviated as FDO or FCO respectively. "CU" and "FDO" may have been shortened in order to fit into the available (and cramped) space.
"De" in Spanish translates to either "of" or "from." In the case of family names, it indicates something to the effect of "I am of xxx lineage."
In other words here, "De Royarn FDO" may indicate that the man in question was the Son of Royan Fernando, the "CU" being an abbreviation of the surname of Father and Son.
However, I may just be full of "mierda" on this one...
