It's a complicated area and I'm certainly not an expert on copyright, but since this is an international forum we generally go by the copyright law in the native country of the composer, Brazil in this case, and my understanding is that copyright there applies for 70 years after the composer's death, regardless of publication date, and that no registration is necessary.
The important thing to bear in mind is that this forum is hosted in France, where copyright is very rigorously enforced. France is a fellow signatory with Brazil to the Berne Convention which guarantees reciprocal respect for the copyright laws of all signatories, so that's the one we need to follow here, even though copyright law is very different (and complex!) in the USA and it's true that things may be in the public domain there when they are still copyrighted elsewhere.
Classical Guitar Forum.
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