Since it's been brought up, there really isn't any detail in this shield that confirms it's the arms of Portugal. There is a bordure, or band around the edge, but the Portuguese arms feature a " bordure of Castile," which has a series of castle turrets along its length. This bordure looks like a deep trench, so I was thinking it might once have been filled with enamel decorated as the bordure of Castile, as coinsoul suggested. The central design should be five tiny shields, arranged in the form of a cross. Of course, the crude colonial issues seem to lack some of these details to varying degrees.
Thanks for the better pics, rooneydog. More food for thought.