Hello all,
Thinking it over, I support the idea that it is a suspicious folded letter and that its authenticity should have more support. For example, X-ray analysis, since this technique can sometimes reveal if there were other stamps under the stamps that were added, because there may be traces of adhesive, or the paper may be slightly wrinkled or altered.
The folded letter was apparently expertised, as it has a small marking (French expert Roger Calves?) in the lower right corner. Guillermo Marroquín was the Judge of Capellanías in mid-1871, according to what I was able to research, but that doesn’t prove anything either, since the folded letter could have been stampless and someone clever might have added the stamps afterward to make a monetary gain. All in all, no real opinion can be given because it’s necessary to examine the paper underneath the two stamps.