Quick answer

The 323.10.42 prefix on an Omega Speedmaster points to the 42mm steel Moonwatch on a steel bracelet, which is the most copied Speedmaster shape there is. If you are shopping for a replica of it, three things decide whether it looks right on the wrist: a 42mm case with a lug-to-lug near 48mm, a matte black step dial with subdials sitting at 3, 6 and 9, and a flat three-link bracelet whose end links meet the case with no visible gap. Get those right and most people will never look twice. Get the crystal or the bezel insert wrong and the watch reads as a fake from across a table.

What 323.10.42 actually refers to

Omega builds its reference numbers in blocks. The first group covers the collection and case material, the next covers case size, and the trailing groups cover movement, dial and bracelet as fitted at the factory. The 323.10.42 opening describes a Speedmaster Professional with a 42mm case and a steel bracelet, which takes in a run of Moonwatch variants that share the same case architecture and a hand-wound chronograph movement.

That matters when you read replica listings. Most sellers shorten the number to "323.10.42" and never mention the last digits, so two watches advertised under the same heading can differ in crystal type, dial finish or bracelet generation. Before you compare two replicas, ask each seller for the full reference being copied and confirm the details line up with the same original.

Case and crystal: where replicas give themselves away

The genuine case measures 42mm across, about 13.5 to 14mm thick, with 20mm lugs and a lug-to-lug close to 48mm. Ask for a photo with calipers on the case, not just a wrist shot. Replicas often come in half a millimeter or a full millimeter over, and that extra meat shows up as a slab-sided profile that sits too tall on a small wrist.

Then check the crystal. Moonwatches of this era shipped either with a domed acrylic (hesalite) crystal or with sapphire. Both are reproduced, and both can be done well. The dome is the harder one to fake: flat sapphire with a light edge bevel on a watch that should have a warm, gently distorted dome is an easy tell. Look for a photo taken side-on.

Caseback

Steel casebacks should have crisp engraving, a straight seahorse and legible text. On cheap replicas the letters are shallow and the medallion edge looks stamped rather than machined.

Dial, hands and tachymeter bezel

The dial is matte black with a step at the outer track, printed text, and three subdials at 3, 6 and 9. Subdial spacing is the fastest dial check, because it is driven by the movement underneath. A chronograph movement with the wrong subdial layout forces the factory to print the registers in the wrong place, and the 6 o'clock register ends up sitting too close to the center.

Hands are simple and unforgiving. Hour and minute hands should reach the minute track without running past it, with clean white lume fill. The chronograph seconds hand has to rest dead on 12 when reset. If a seller's video shows it parked at 11:58 and staying there, walk away.

Bezel inserts are aluminum on the originals, with a printed tachymeter scale. One detail worth researching: earlier and later Moonwatch bezels differ in whether the dot sits over the 90 or diagonally to it. Pick the correct variant for the original you are copying, then check the replica against that photo.

Bracelet and clasp

The bracelet is a flat three-link design, brushed on the outer links, 20mm at the lugs, tapering toward a folding clasp with a stamped logo. Things to inspect in photos: end links that sit flush against the case with no step, pins or screws that sit flush in the side of the links, and clasp engraving that is sharp rather than fuzzy. Ask how many removable links come with it. A 42mm watch on a bracelet that is two links short is not a bargain at any price.

Inside a replica: what you are actually buying

Most hand-wound replica chronographs in this shape run a Seagull ST19 family movement, a Chinese column wheel chronograph. A few use other Asian hand-wound chronographs. Here is the part buyers find surprising: the ST19 beats at 21,600 vibrations per hour, the same as the original hand-wound caliber, so the sweep of the seconds hand is not a useful test of anything. Neither is the sound.

What you can test is function. Start, stop and reset the chronograph several times. The pushers should have a firm, defined click, the seconds hand should snap back to 12 every time, and the movement should not stall when the chronograph runs for a few minutes. Ask for a video, not a promise.

How to vet a listing before you pay

• Caliper photo of case diameter, thickness and lug width
• Straight-on dial photo in flat light, so printing and subdial placement are readable
• Side-on shot for crystal dome and case profile
• Short video of the chronograph starting, stopping and resetting
• Clear photo of the clasp and caseback engraving
• Written confirmation of what happens if the watch arrives dead or wrong

Sizing, wear and expectations

At 42mm with a 48mm lug-to-lug, this watch wears close to its measurements. On a 7 inch wrist it looks right; under 6.5 inches it can overhang. The 20mm lug width means straps are easy to find, and a leather or nylon strap is a good way to make a replica look less like a replica, since the bracelet is often the weakest part.

Keep it out of water no matter what the listing claims. Treat it as a mechanical watch that needs service every few years, and expect to pay a watchmaker who works on Asian chronographs. One more thing worth saying plainly: replica watches are restricted or illegal to sell in some countries, and passing one off as genuine is fraud. Know the rules where you live before you buy.