A Lemania base in a Patek case
For decades Patek Philippe built its automatic chronographs on movements supplied by Nouvelle Lemania, the SSIH-era Swiss specialist (now part of Swatch Group as Manufacture Breguet). The Cal. 27-70 is one of these: a heavily-modified Lemania 2310 derivative, the same architectural family as Omega's Cal. 321 and many other classical column-wheel chronographs of the 1940s-1990s. Patek added its own bridges, chrono module refinements, and finishing.
The Nautilus 3712: the first Nautilus chrono
The Cal. 27-70 is most famously associated with the Nautilus ref. 3712/1A (2005), the first Nautilus chronograph and a transitional, short-lived reference produced for only about a year before being replaced by the in-house Cal. CH 28-520 in the new ref. 5980. The 3712 is now a cult collectible: only ~1,500 were made, the dial layout is unique (small chrono counter at 9, power reserve at 12, moonphase at 6), and it represents the last chapter of Patek's Lemania-base chronograph era. Auction results for the 3712 routinely cross USD 200,000-400,000.
Why Patek moved away from Lemania
By the early 2000s the watchmaking landscape had changed: Swatch Group consolidated Lemania, Nouvelle Lemania, and other historical chrono manufactures, raising the question of whether independent buyers like Patek could continue to depend on outsourced movements. Patek made a strategic decision to develop its own automatic chronograph in-house, resulting in the CH 28-520 in 2006. The Cal. 27-70 became the last Lemania-based Patek chronograph movement, and the 3712 the last Nautilus to wear one.
The 27-70 in earlier Patek chronographs
Beyond the 3712, the 27-70 architecture (and closely related variants 27-70 PS, 27-70 Q for perpetual calendar) powered several earlier Patek complications references through the 1990s and early 2000s, including some perpetual-calendar chronographs in the 3970 family and certain limited variants. The base architecture is the same: column-wheel, lateral clutch, classical chronograph layout. Patek's contribution is in finishing (Geneva Seal-grade, hand-bevelled bridges, polished sinks) and in the complication modules layered on top.
Where it sits
Today the 27-70 is a transitional moment in Patek history. Before it, Patek depended on Lemania; after it, Patek built its own. The 3712 is the watch where this pivot happened, captured in a single short-production reference. For collectors of vintage and neo-vintage Patek, the 27-70 is the engine that defines an era; for owners of the modern Nautilus 5980, it is the predecessor that made the in-house CH 28-520 necessary.