After examining a large number of these stamps, my assessment is that there are at least three overprint plates over the small format stamps. The scarcest is the 10c Rivadavia Red, which seems to have seen postal use mostly in October and November of 1938. These two overprint varieties are unique to this plate.
"S" of "Servicio" with ink blot.
Ink blots along top edge of "Servicio."