This is a fine Patek Philippe Ref. 3418 Calatrava, also known as the 'Convertible' This reference was believed to only be made in stainless steel for a hand full of years. This rare and unusual model allowed for easy strap/bracelet changes by unscrewing a metal ring fixed to the case back and revealing the lugs within. Usually these watches have the lug pieces missing, but this watch has them which enables easy switching from strap to bracelet. This piece comes on its original mesh Patek Philippe bracelet that closes like a belt buckle. Just such a cool invention all around. The Calatrava style watch is powered by a manual wind caliber 12-400 18 jewel movement. The original silver dial is pristine with nice and simple slash markers and a subsidiary seconds register at 6 oclock.