This is a crytopgram, a signature where the abbreviations stand for a bigger message. There is no full consensus about its meaning. A common interpretation has been: "Servidor; Sus Altezas Sacras; Jesus (the "X" is a Greek abbreviation for Christ), Maria, Ysabel; Cristoforo, El Almirante." Or "Servant of; Their Sacred Highnesses; Jesus Mary and Isabella; Christopher; the Admiral."
This rare signature belongs to Christopher Columbus. This subject of his signature came up in conversation after a friend of The Raab Collection returned from Seville in Southern Spain, home to Archive of the Indies, which houses the world's known supply of Christopher Columbus historical documents.