The 1518 was Patek's first Perpetual Calendar Chronograph when introduced in 1941. While commonplace -though still a mean feat today, at the time it was nothing short of spectacular. In a case of just 35mm, Patek managed to squeeze in a hand-wound chronograph movement with small seconds, displaying also the day, date, month and moonphase.
Some four examples, out of a total of 281, of the 1518, are said to have been produced in stainless steel. The latest public sale of a stainless steel 1518 was at a Phillips auction in November of 2016. Sold at CHF11,002,000 including fees, it became the most expensive stainless steel watch to have ever been sold at auction.
The price graph shown is an indication based on various sources around the web; they should be considered indicative at best.