Looking at this cannon, we notice that the shield is oval, that of Dom Joćo while Prince Regent, as it says in the inscription. Later when he became King (and Emperor) while in Brazil, we see that the Coat of Arms becomes circular, including the armillary sphere. After 1826 it returned to the prior version, as by then Brazil was already independent. In any case, your halberd having the circular shape but not including the armillary sphere, could be due to it not having been made in Brazil but in Mozambique ... i would venture.
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