Calling all Rolex lovers and collectors. A unique set of “tropical dial” timepieces is heading to the auction block, and you can participate online, in mid September.

Bob’s Watches is a U.S.-based online reseller that prides itself on being the world’s first and only pre-owned Rolex exchange offering consumers a space to buy, sell and trade vintage and pre-owned Rolex watches at true and fair market values.

See also: Inside Staple Pigeon Founder Jeff Staple's Historic Fossil Watch Design: 'An Heirloom Timepiece'

Come September 8 to 14, Bob’s Watches will host a unique collection of “tropical dial” Rolex sports watches, including lineup of pieces from the ‘60s and ‘70s.

Rolex ref. 1675 GMT-Master from 1969

“We are happy to offer these four, rare and beautiful watches with tropical dials,” says CEO and Founder of Bob’s Watches, Paul Altieri. “‘Tropical dial’ refers to when the dial’s color changes, and in this case the dials have gone from black to brown, [which] are highly sought after.”

The upcoming lot features a Rolex ref. 1016 Explorer from 1966. Originally black, the glossy dial aged to a rich brown, earning the timepieces an estimated value of $30,000 to $50,000.

Rolex ref.1665 “Double Red” Sea-Dweller, circa 1972

The Rolex ref.1665 “Double Red” Sea-Dweller, circa 1972, boasts a matte black “Mark II” dial that’s turning brown around the edges and the date window. With a value estimated between $50,000 to $80,000, it is the most sought-after watch up for grabs.

Also up for grabs is a Rolex ref. 1680 “Red” Submariner, from 1970, which comes with its original box and punched papers. Its original black meter’s “Mark II” dial has turned completely brown, while the evenly faded bezel turned from black to greyish-blue. It should fetch somewhere between $20,000 to $40,000.

Rounding out the offerings, fans will find a Rolex ref. 1675 GMT-Master from 1969. The “Mark I, Long E” dial has changed from black to brown, paired with a fuchsia bezel. It has an estimated value of $25,000 to $45,000.

Rolex ref. 1016 Explorer from 1966

“These are all fresh to market watches and each is one-of-a-kind, because no two tropical dials age the same exact way,” says Director of Vintage, Brandon Frazin. “We are passionate about vintage watches and excited to share this collection with the world.”

Those looking to place their bids should register on Bob’s Watches’ website and mark their calendars for the auction. Each bidder will be notified if they have been outbid and given a chance to up the ante to secure their choice.

Winning bids will be announced on the website after Sept. 14. Learn more at bobswatches.com.

Rolex ref. 1680 “Red” Submariner, from 1970