Information received from Central Bank of Brazil:
Dear Niels,
There was a recent decision in Brazil, which allows us to import coins.
We decided to identify the coins by international supplier, with letters, in alphabetical order.
RDM is the first supplier and was assigned the letter "A".
We also decided to not include a supplier identification on the coins produced in Brazil.
So we will likely in the future also see B, C, D etc. Strange that they did not just allow the mints to use their own mintmark, but I assume they may not be aware that many mints already have a specific mark.