This stamp is one of the 1935-51 definitives. There are other 20 centavos issues in this series: the 20c small format cattle definitive, and the 20c Guemes definitive. This stamp saw most of its use during World War II. There are three separate pages associated with the 20cLC issues: one dedicated to plate varieties and printing shifts between the two plates used, and another dedicated to postal usages. Below is the first 20cLC issued, the 20cLC1E, on watermarked cream paper with the first watermark.

Below is the second 20cLC issued, the 20cLCNGR, on unwatermarked gray paper with a grid pattern.

Below is the third 20cLC issued, the 20cLC2, on watermarked bright white paper with the second watermark.

Recommendations for further browsing:

I have a page with many covers of this printing as well as the later printings of the 5 Pesos Iguazu Falls.

The continental grain correspondence page contains several 5 Pesos Iguazu Falls covers.

The Argentina 1935-51 issue pages from the 5 pesos types page there is additional information about each of the printings.

Also, an early article dedicated to a lower value in the series, Article 5: Issues of the 20c "Large Cow" (20cLC) Value.