"Through the 1940s, most Patek Phiippe buckles looked the same. ... By the 1950s, a new style of Patek Phippe buckle, designed by Henri Stern himself, emerged. ... This particular design was used exclusively in the United States. ... In the 1980s, this buckle became standard for all Patek Philippe strap watches."
Abstract from "Patek Philippe in America"
By the way, one more beauty you have there 
Oliver