Introduced in 1985, reference 3940 quickly became one of Patek Philippe’s most iconic models, remaining in production for over two decades until it was succeeded by reference 5140 in 2006. Scholars have divided the model production into four distinct series, with a pre-series 25 pieces limited edition created to celebrate the 225th anniversary of the well-known retailer Beyer (these rare models feature a distinctive gilt dial with the series number individually printed).The four series have been categorized as follows:-First Series: Characterized by sunken subsidiary dials as well as a solid case back. -Second Series: Introduced the beveled subsidiary dials, and equipped with a solid case back with additional sapphire back available.-Third Series: Introduced a cross to separate the leap year indication, and equipped with a sapphire case back with additional solid case back available. -Fourth Series: Characterized by the "σ Swiss σ" writing aligned with the minute track dots (the present example).