Demonetization

 These dates are from Pettigiani:

January 2, 1937: 1pL

January 8, 1939: 3cGr, 4cGy, 10cR, 12cBR, 15cSC-D, 20cJMG, 20cMG-L

January 4, 1941: 2 1/2c

June 30, 1944: 15cSC-L

January 9, 1957: 1/2c, 2c,3cGy,3cSM, 4cGr, 5c, 6c, 8c, 12cR, 15cMG, 20cMG-D?, 20cLC, 20cSC

January 6,1959: 25c, 30c, 40c, 50c, 1p, 2p, 5p, 10p, 20p

February 28, 1962: 10cBR

September 30, 1963: 1c

The cover below circulated four days before demonetization of the 10 centavos Rivadavia Red (10cR).

Two 1938 Covers to Germany

 The first one is an air mail cover, and the second one was sent by ship.

Two 1938 Covers to Rare Destinations

 The first is to the Canal Zone, which is common as a stopping point to North America, but not as a final destination. The second one is to modern-day Israel, then Palestine.

Two 5 pesos 1938 Covers

The first one is to France, and the second one to Germany.

Two 1938 Covers to Colombia

 To the same address, and with commemoratives.

Two 1938 'cents' Covers

Both look like phitalelic uses. The second one has an 'opened for the purposes of currency controls' label strip on the back.

1938 Covers (One to Czechoslovakia, One to England, Three to France, )

Imagine living in Czechoslovakia, and a year later the country is occupied by Nazi Germany... World War II.