Quick answer
On a replica Datograph 912.021 you are buying a dial layout, not a movement. The big date, the two subdials, the case shape and the strap get copied well enough to pass at arm's length. The chronograph inside does not get copied, because the real watch runs an in-house column-wheel calibre with a flyback function and hand-finished parts that no rep factory produces. So ignore the spec sheet and judge the watch on the dial, the pushers and the case finish.
What the 912.021 number tells you
Reference numbers in the Datograph family get quoted loosely by sellers. Some listings use them to point at a specific metal and dial combination. Others paste a number onto whatever case happens to be in stock. I treat the number as a marketing line until I see it engraved on a caseback in a clear photo, and even then it says nothing about the inside. If the catalog reference matters to you, compare it against the reference list published by the maker or a reputable dealer before you pay.
Movement: what is actually inside
The genuine Datograph earns its reputation from a hand-wound chronograph with a column wheel, a flyback and finishing you can inspect through a sapphire back. Replicas do not have that. What you will find instead:
- Seagull ST19 family. Hand-wound, column wheel, closest pusher feel to the original. Finish is machine grade, servicing is cheap, parts exist.
- Valjoux 7750 based automatic. Common and robust, but the pusher feel is softer and the subdial positions rarely match the original layout. It cannot do a true flyback without added parts.
- Quartz. Accurate, silent, wrong. The seconds hand ticks once per second and the subdials are often decorative.
If a seller claims a Swiss flyback movement at a rep price, ask for video proof. Start the chrono, then press the lower pusher once while it runs. On a true flyback the hand snaps to zero and keeps running. On most reps the chrono just stops, or nothing happens at all.
Dial checks
The outsize date is the first thing I look at, and it is where most reps fall apart. Two discs should sit level with each other, with no step between them and no dark gap at the top of the digits.
- Digits: equal weight in both windows, sharp edges, no fuzzy paint.
- Alignment: centered in the aperture, no tilt, no cut-off numerals.
- Date change: watch it roll over. Cheap builds show a blank window for hours or skip two numbers.
- Subdials: small seconds sits low near 8 o'clock, chrono minutes near 4. Hands should reach the track instead of stopping short.
- Printing: thin and crisp. Blobby text, thick logos and drifting markers mean a low tier build.
Case, crystal and pushers
First-generation Datograph cases measure 39 mm across. Later Up/Down versions run 41 mm. Measure before you buy, and do not trust a listing that describes the size as large or standard.
- Crystal: sapphire with a slight dome. Anti-reflective coating should show a faint blue or purple tint at an angle, not a rainbow mirror.
- Pushers: two-stage click on start and stop, a clean snap on reset, no grinding.
- Bezel and lugs: crisp line where brushed surfaces meet polished chamfers. Blurry transitions are the classic tell.
- Caseback: engraving depth, font and spacing. Shallow laser etching looks flat in photos.
- Strap and clasp: leather that does not crack in a month, and a deployant that does not chew the strap.
Before you pay
- Ask for a video rather than photos. Start, stop, reset, plus a flyback attempt if the listing claims one.
- Ask for the case measured with calipers.
- Ask which movement sits inside and who can service it.
- Confirm the return window and check your country's import rules before ordering.
Two honest notes
A replica is not a genuine watch and it will not hold value. Do not swim in one, and assume any water resistance claim above splash level is optimistic. Selling a replica as the real thing is fraud in most places, and importing one can be a customs problem depending on where you live. Buy it as a watch you enjoy wearing, and keep the paperwork for what it actually is.