Most mariners' astrolabes are the products of either Spain or Portugal (a tiny proportion have been attributed to British, Dutch or French makers) where the industry became highly evolved with, according to Stimson, seven distinct patterns emerging between 1500 and 1700, which is believed to be their approximate period of use at sea - this example is Type I(a), a cast wheel with base ballast. Early in the sixteenth century, the Spanish authorities required all navigational instruments to be examined by the Pilot Major of the Casa e Contratación for accuracy, with successful instruments being stamped with an approval mark. The regulation was much ignored, to the extent that it was re-issued in 1545, however it is presumed that in the period when Portugal was a province of Spain (1580-1640) these regulations also applied, and the armillary sphere (a symbol adopted from King Henry the Navigator, 1394-1460, by Manuel I, 1469-1521, and still used on the Portuguese flag) was used as the approval mark.90°- 0°- 90° markings: Portuguese astrolabes measured zenith distance, the complement of altitude, and thereby reducing the latitude calculation by one stage by eliminating the need to subtract the observed altidude from zero