The reference 1518 played a critical role in the development of Patek Philippe's complicated wristwatches. Together with reference 1526, it was the first perpetual calendar wristwatch ever produced by the company in series. It is recognized that the introduction of reference 1518 greatly contributed to the manufacturer's dominant position in the Swiss watchmaking industry. Indeed, perpetual calendar wristwatches produced in series were almost exclusively a Patek Philippe commodity for four decades (other manufacturers tending not to list or distribute the complication other than for special orders).
The influence of reference 1518 can be seen in all the series produced Patek Philippe perpetual calendar chronographs that came after it (2499, 3970, 5970, 5270). Even though the case design changed in the references which followed, the basic dial layout remained the same. In the same way, the functions of the movement have not changed in 70 years.
The 1518 was equipped with a Valjoux calibre 13Q (13 130Q) which was first upgraded and finished to the highest standards for the time in Patek Philippe's workshop. Then, Victorin Piguet was employed to modify the under dial work and incorporate the perpetual calendar function. It was not until 1985 that Patek Philippe replaced the Valjoux for a Lemania calibre in its perpetual calendar wristwatches. With the new Ref. 5270, the Lemania caliber was replaced with the fully in-house caliber CH-29-535 PSQ.
From 1941 to 1954, a total of 281 reference 1518 wristwatches were made, an incredibly small production run by today's standards and an average of just 20 watches a year.
Similar examples are illustrated in Huber M., Banberry A., Patek Philippe Wristwatches, 1998 edition, p. 303.