Perpetual Calendar from H. Moser & Cie

Courtesy of H. Moser & Cie

Not 40, not 41, but a cunning 40.8 mm - I love the refusal to round the case size the nearest millimeter!
In terms of the mechanism, the Perpetual Calendar utilizes what H. Moser & Cie refers to as a Flash Calendar.  Whereas many date mechanisms gently glide into position for the new day, the Flash Calendar essentially changes in the flash of an eye. maintaining the correct date - even in months that are shorter such as February.

Here are some more details - click on the hyperlinks to get the details of the various functions ; )

Reference 341.501-001 - Hand wound movement
Rose Gold
40.8 mm in diameter

Case : 40.8 mm diameter, 11.1 mm height
Movement: in-house hand-wound calibre HMC 341,
34.0 mm diameter, 5.8 mm height
Frequency: 18,000 vib/h
Perpetual Flash Calendar display
Centre hand month indicator
Hour and date
Off-centre seconds dial
Power-reserve indicator
Hacking seconds
Double Pull Crown mechanism
Double barrel
Moser teeth for all wheels and pinions
Interchangeable Moser escapement
Original Straumann Hairspring® with stabilised Breguet overcoil
Pallet fork and escapement wheel made from gold
Movement and components hand-finished and decorated
Leap year cycle indicator visible on movement side
See-through sapphire crystal case-back
Hand-stitched alligator strap
Solid gold or platinum folding clasp with an engraved Moser logo