Argentina 1935-51 12 centavos value

From the specimens and usages available, it is unclear to me which postal rates were intended. The 12 Centavos Brown and Red issues are found on various philatelic covers. Here are quick links to the "cents" section, the 12 centavos design page, the 12 centavos issue page, the 12 centavos plate varieties page, and the "cents" covers main page.
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