Jopie,
You have a nice First Day of Issue cover for those stamps, Scott 927 and 928. The stamps depict early Brazilian stamps issued in 1861 (Scott 39 and 40)
I have heard the early issues nicknamed "Olho de Cabra" because they were printed in black with white numerals and without any legend and are said to resemble the eye of a goat.
I have also heard them called "ohlo de boi, " eye of the ox. I think the difference had to do with their values. I'm not sure though.
Now having said that, Scott 39 and 40 weren't black but red and yellow. So go figure.

Maybe I'm confused on the color issue.
The maps on the right of the stamps are probably the reason for the other symbolism on the cover. How that ties in is way beyond my knowledge of Brazilian History.
I hope that's a start.
Dan