The reference 3541 was produced between 1961 and 1967 and is referred to as a "Monodate" featuring an automatic movement with subsidiary small seconds and a date aperture. It was predominantly cased in yellow gold and occasionally in white gold. 

SPECIFICATIONS 

Brand: Patek Philippe
Model: Calatrava 
Reference Number: 3541J
Movement: Automatic
Gender: Men's
Condition: Very good (worn with little signs of wear)
Time Period: 1966
Box: No
Papers: Extract from Patek Philippe Archives

THE CALIBER

Caliber: 27-460M
Jewels: 37

THE CASE

Case Material: 18K yellow gold
Case Diameter: 36 mm
Thickness: 10.5 mm
Waterproof: 
Bezel Material: Fixed 18K yellow gold
Glass: Acrylic (apparent scratches on the glass that can be eliminated by polishing the crystal)
Caseback: Solid
Dial: Silver with radial brushing 
Dial Numerals: Yellow gold applied baton hour indices, outer perlé minute track around the outer rim

THE BRACELET / STRAP

Bracelet Material: Black Leather
Buckle: Tang
Buckle Material: Gold plated

THE FUNCTIONS

Hours, minutes, guichet date at three o’clock, subsidiary seconds dial at 6 o'clock

FURTHER READING

Yellow gold applied stick-shaped hour/minute hands small second hand
Solid two-body round case and bezel are polished/brushed, excellent condition
Solid 18K yellow gold screw-down case back engraved with reference number
Sharp straight 18K yellow gold angled lugs with a crisp and visible hallmark in the back
18k yellow gold pull/push recessed knurled crown with Calatrava cross engraved
The caliber 27-460 is stamped with the Geneva Quality hallmark, and it has the Patek Philippe seal. It's rhodium-plated, with fausses cotes embellishment, a shock absorber device, and a straight-line lever escapement. It includes a self-adjustable balance spring stud carrier, an adjustable-mass balance with a Breguet overcoil spring, a Gyromax balance adjusted for heat, cold, isochronism, and 5 positions a centrally mounted solid 18K yellow gold rotor, consummately decorated with the engine turning.
This mechanism oscillates at the frequency of 19,800 vph (3 Hz). 
The caliber 27-460 M, the successor of the caliber 12-600 AT, was the first automatic watch movement produced by Patek Philippe in a series in production for 25 years, from 1960 to 1985.
This caliber 27-460M offers the same exquisite finishing as the 12-600 AT movement in the ref. 2526.
Walt Odets called Cal. 27-460 "the reigning queen of that half-century" and "the most elaborate and costly simple automatic watch movement ever produced."
Caliber 27-460 M was also called "La Grande Dame des automatiques Suisse"
Incomplete (no box, with an extract from the Patek Philippe Archives, dated 1966)