Argentina 1935-51 Definitives 20 Pesos on P.O. Box Receipt

The Argentina 1935-51 20 Pesos is one of the great high value definitives of the 20th century. The early printings are extremely rare on cover, and the later printings were in use when the stamp was still a mid-range high value.
Below is a Post Office Box Receipt with a vertical pair of 20P1E's. This is a very late usage (1952), which makes me wonder if there was a printing in the 1950s with the same colors as the 20P1E.
Below are details of all of the stamps, and both postmarks. The purple postmark is barely readable, although I can read "RIVADAVIA" from it.

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