Basic Guidelines For English Spellings
READ THESE ARTICLES
In the 18th century: a Spanish ship with a special licence authorizing it to trade outside the fleet system with the Spanish possessions in America, in specified areas or countries.
The cargo of a register ship had to be specified to the Board of Trade, the body controlling all Spanish trade with the West Indies, according to both nature and volume.
Early 18th century; earliest use found in Account Cruelties by Inquisition in Portugal. From register + ship, after Spanish navío de registro.