One of the world’s most unique and complicated watches—the white gold Calibre 89 by luxury Swiss watchmaker Patek Philippe—is going on the auction block April 24 in Geneva, Switzerland. A yellow gold version sold at auction in 1989for $3.2 million, one of the highest prices ever paid for a timepiece.

The Calibre 89, an open-faced astronomical clockwatch with two dials, has 33 complications in five categories of functions: timekeeping, calendar, chronograph, chime, and operational functions. There are only four models—one each in yellow, pink, and white gold, and one in platinum.

The Calibre 89 was built by a team of watchmakers at Patek Philippe & Cie in Geneva to mark the company’s 150th anniversary. The entire project took nine years. Preliminary calculations and designs began in 1980, a working prototype was ready in 1988, and the watch was completed in 1989.

The yellow gold Calibre 89 was auctioned by the Antiquorum auction house on April 9, 1989, in a sale commemorating Patek Philippe’s 150th anniversary. It and the other three pieces were purchased by an unidentified royal family. In recent years, the collection has been broken up. The yellow gold watch was bought by a Japanese watch and automobile collector, and is now in his private museum. The pink gold model was sold to an Italian collector, and the platinum version was bought by a Middle Eastern royal family for their private museum. The white gold watch is the last piece to be offered for sale.

JCK