The Seiko Model 5 probably is the best watch you've never heard of. Seiko sold more than 5 million of them in the four years after its introduction in 1963. Never had so many paid so little for a high quality watch. Critical parts were machined to within 2 microns (1/25,000th inch) of spec, a tolerance not typically seen in a $20 watch. Here, at last, was the poor man's Rolex.

Like its luxury-oriented Swiss counterpart, the Model 5 had a metal band and an automatic movement that eliminated the daily winding ritual. It was also water resistant and ridiculously over-engineered: The mainspring was made from a nearly unbreakable alloy called Diaflex; an ingenious shock absorption system protected the movement from violent impact; and the patented Hardlex crystal was virtually shatterproof. More than just being rugged, the Seiko 5 had style. The stainless steel lugs were brushed, the day and date were displayed in the same window (a novel idea in those days), and the crown, recessed behind the 4 o'clock position, gave the case a clean look.

A half a century later, this Japanese classic remains in production. And a growing community of Seiko modders has made it much more than the poor man's Rolex. It's also the poor man's Tag Heuer, Panerai, Omega, and a bunch of other expensive brands most people can’t afford. These "homage" Model 5s are attractive in a way black market counterfeits never can be. For one thing, they’re made to last. The surprisingly accurate 7s25/6 and 7s35/6 calibers that power these affordable timepieces—some new models sell for under $100 on Amazon—are workhorses that can go decades without maintenance.

Although they frequently are mistaken for luxury watches, these Seiko mods are tweaked just enough to have their own cheeky personalities. Subtle details that set them apart from their pricy doppelgangers can include hand variations (Mercedes, snowflake or military), glass bezel inserts, domed sapphire crystals, day/date discs and dials that look like the real deal, sans logo. Wearing one is like bringing a respectable $20 bottle of wine to a formal dinner party and having the filthy rich host mistake it for Pétrus.

A Seiko Model 5 masquerading as an Omega Planet Ocean. Harold Ng

What makes this particular watch so hackable? In a word: numbers. Like Lego kits, the supply of Seiko 5s is inexhaustible, the variety is great, and all the components, including the movements, are interchangeable. Add a cottage industry specializing in aftermarket parts, and the tweak factor rises exponentially.

Consider the Fifty Five Fathoms (FFF), a modified Seiko 5 that bears an uncanny resemblance to the highly collectable Blancpain Fifty Fathoms, a legendary diving watch made famous by Jacques Cousteau and the US Navy Seals. Or perhaps you lust over the Omega Planet Ocean that Daniel Craig wore in Skyfall. There’s a Seiko mod for that, too. Other popular homage watches include the Rolex Submariner (satin bead blasting optional), the Rolex Explorer and the über Teutonic Sinn 656. There’s even a Seiko 5 mod that pays tribute to the Seiko Marine Master Sbdx011, a 1,000-meter diving watch that will set you back $2,600. Known within the mod community as the PMMM (poor man’s Marine Master), its wrist presence is every bit as formidable as the original. But be forewarned; rated at 200 meters, it won’t survive the Mariana Trench. Mash-ups, of course, are rife. The Planet Monster is a Dr. Moreau creation that melds the DNA of a Seiko Monster and an Omega Planet Ocean.

For those who are either dexterously challenged or don’t relish the idea of sourcing dials and tools—not to mention pouring over tedious YouTube tutorials—professionals can expedite your horological dreams. The most respected guru within the Seiko 5 modding community is Harold Ng at Yobokies (that’s SeikoBoy spelled backwards). Go to any watch forum worth reading, and his name will come up when the discussion turns to FFFs and PMMMs. Just peruse the Yobokies photobucket account, and drop SeikoBoy an email. Spend the cash you save not buying a real Rolex on upgrading your wardrobe.