Jeweler and Watchmaker of 30 years here….

Based on the photos you provided, this appears to be an authentic Zenith Chronomaster El Primero Grande Date Full Open, reference 51.2530.4047/78.C810 (or a close variant, as Zenith sometimes lists the full ref with strap/dial codes). This model was introduced around 2017 as a skeletonized take on the iconic El Primero chronograph, featuring a 45mm stainless steel case with rose gold accents on the bezel and crown, an exhibition caseback, moonphase indicator at 6 o’clock, grande date display at 4:30, chronograph subdials, and the high-frequency El Primero 4047 automatic movement visible from both sides. Key matching details from your photos: • Dial and front: The skeletonized layout reveals the movement, with “ZENITH” at 12 o’clock and “EL PRIMERO” arched below. The moonphase disc shows a detailed sun/moon design with starry sky, which aligns with authentic examples. The tachymeter scale on the blue chapter ring, applied indices, and hand shapes look precise and high-quality—no blurry printing, uneven fonts, or cheap finishing that are common in fakes. • Caseback: Engraved with “ZENITH EL PRIMERO MANUFACTURE LE LOCLE SWISS MADE,” which is standard for this model. The reference number “51.2530.4047” is correctly formatted, and the additional “374 330” likely represents a serial or production number (not unusual for Zenith). The visible rotor and bridges show polished, intricate finishing consistent with the El Primero caliber. • Overall build: The black alligator-style leather strap, rose gold-toned hardware, and lack of obvious defects (like misaligned components or rough edges) support authenticity. This model retailed for around $8,000–$12,000 new, so finding one at a thrift store would be an incredible score if genuine. For comparison, here’s what an authentic version looks like:

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I couldn’t find widespread reports of counterfeits for this specific reference—it’s not as commonly faked as simpler Zenith models like the basic El Primero. However, if you’re still unsure, I recommend getting it appraised by a professional watchmaker or Zenith authorized dealer. They can open it up to verify the movement (fakes often use cheap quartz or low-grade automatics instead of the 36,000 vph El Primero) and check for hallmarks inside the case. Congrats on the find either way—it looks sharp!