Here begins my first stab at grouping these stamps in a way that can help us identify all of the printings in this complex series.
This group is the first batch of "cents" issues. I am also including the 10c typographed. All of these stamps can be found on philatelic covers from the exhibition that took place in Buenos Aires in late October 1935.
The stamps are:
1/2c, 1c, 2c, 3c San Martin Green, 4c Gray, 5c offset, 6c, the various typographed printings of the 10c Rivadavia Red, the 12c Brown, and the two variations of the 20c Martin Guemes (Juan Martin Guemes-or JMG, and Martin Guemes-or MG). I am also including in this group the 20c Martin Guemes, even though it was issued sometime in 1936. This is a scheme in progress....
Here is a second example: