The Ref. 5350 Annual Calendar in a limited "Patek Philippe Advanced Research"is a special edition and was the first Patek Philippe watch with a Spiromax spring.
The technology used to create this new type of balance spring resulted in 3 patents being granted to Patek Philippe. The Spiromax balance spring is made from Silinvar, a new compound derived from monocrystaline silicon. Patek's engineers devised a new terminal curve design for the balance spring taking full advantage of the properties of Silinvar. This meant shifting the mass of the spring for perfect symmetry as it expands and contracts over its entire spectrum of movement. This new balance spring has far improved isochronism and eliminates the need for lubrication of yet another critical part of the balance assembly. To do justice to these technical innovations, the movement has a specially shaped balance cock that reveals a large portion of the Spiromax spring, and the going train bridge is milled so that the blue-violet sheen of the nearly black silicon escape wheel is more readily visible. Additionally, the sapphire-crystal back has an integrated loupe and bears the inscription "Patek Philippe Advanced Research."
The solid 21K gold winding rotor features a zirconium ball bearing. Like the silicon escape wheel, the balls run dry and never need to be lubricated.
Ref. 5350R was produced in a limited edition of 300 watches.