The Zenith El Primero Chronomaster XXT Open Power Reserve 45mm (frequently designated under reference series like 03.1260.4021) is a bold, historically significant luxury timepiece originating from the Thierry Nataf era of Zenith in the mid-2000s. Known for its substantial 45mm diameter, avant-garde open-heart dial, and legendary high-beat movement, it remains a distinctive choice for luxury watch enthusiasts.

• The "Open Heart" Cutout: Located between the 9 and 11 o'clock positions, the dial is purposefully paired down with skeletonized bridges to expose the escapement, balance wheel, and hairspring assembly in high-beat motion from the front. 

• The Open Back: True to its name, a scratch-resistant sapphire crystal display back reveals the intricate execution of the movement from behind, showcasing a rotor decorated with circular Côtes de Genève, engine-turned perlage, and contrasting blued screws.

• Baroque-Style Case Architecture: The high-polish case stands out due to its fluted, tiered lugs and a triple-stepped "triple gadroon" bezel, projecting an Art Deco aesthetic. 

• Complex Dial Patterning: The face features multiple finishes including concentric circular graining, applied Roman numeral indices, and a stamped radial guilloché pattern. 

• Constant Seconds Functionality: Because the cutout removes the traditional small-seconds sub-dial, a unique three-spoked pinion extension attached to the fourth wheel serves as the constant running seconds hand inside the open window.

• Case Dimensions: The watch features a 45mm diameter case, measuring 14.2mm in thickness and 53.5mm from lug-to-lug, giving it an imposing and highly legible wrist presence. 

• The Movement: Powered by the automatic Zenith Caliber El Primero 4021, a legendary column-wheel integrated chronograph movement composed of 248 parts, operating at a signature high frequency of 36,000 vibrations per hour (5Hz). 

• Power Reserve: Delivers a 50 to 52-hour power reserve, tracked visually via a dedicated fan-shaped indicator on the dial face. 

• Water Resistance: Standard models offer a basic splash-ready 30 meters (3 ATM) of water resistance.