I have just received a small lot of blocks of four and some singles of these stamps. The blocks of four are of the 10c Rivadavia Brown and 15c San Martin.
From the ones I have examined previous to receiving this lot I am postulating that there are several overprint plates on the small format, and probably also several overprint plates on the large format.
I have the following scheme in mind:
1. First range of printings, perhaps with unique overprint plate, from 1938 to the early 1940s.
2. Second range of printings, with overprint varieties unique to this plate that if found on the first printing would prove that the same plate was used for the overprints. Here is an example of a plate variety unique to this range of issues. There are several easy-to-find varieties like this one with peculiar L's.
3. Third range of printings, with unique overprint varieties and unique font.
In reference to the large format stamps, there may be only one plate for the wide overprint and another for the narrow overprint.
There are four placings for the horizontal (40c/2p) format with wide overprint:
and two placings of the 1p/5p format with wide overprint:
and two placings (2p, no scan, horizontal, and 25c/50c vertical format) with narrow overprint:
Its only a curiosity to determine that there are different plates if each issue only has one plate, but it would be interesting if there was more than one plate for some of these issues.