I eventually found this reference:
"Among other taxes, Alden mentions the creation of a quinto (fifth) on cattle hides at the end of the 17th century, the collection of a 10% ad valorem tax on imports between 1699 and 1719 established by maritime municipalities to support local military garrisons, a **guard tax** levied by the municipality of Rio de Janeiro for the
“maintenance of the guard ship off the Brazilian littoral.”
http://209.85.229.132/search?q=cache:xvHh56MM14gJ:www.econ.p... Page 15, footnote
This seems to corroborate what Floriana was saying.