The stamp inside the tank is the whole answer key

Where to look

1. Underside of the tank lid. Many manufacturers stamp or ink the model number and a date code here. Set the lid flat on a towel — it is the most commonly broken part in the bathroom.
2. Back wall of the tank, above the waterline. The most common spot: raised or stamped characters in the china. A phone flashlight held at a low angle makes shallow stamps readable.
3. Inside the tank walls. Some brands put a paper or foil label on an inner wall instead of a stamp, especially on newer tanks.
4. Between the seat bolt holes (the bowl). A few manufacturers stamp the bowl separately. The bowl number and the tank number are both useful, but it is the tank number that keys the parts inside the tank.

What you are reading

• Model number. The money characters. Kohler uses a K-number (e.g. K-4634 for a Wellworth tank), TOTO uses a CST/MS number, American Standard a four-digit series, Glacier Bay a label number tied to Home Depot's parts channel. This number maps to an exact parts diagram in the manufacturer's documentation.
• Date code. Often a year and month, sometimes a week number, stamped near the model. It is not decorative — see below.
• Flush volume. Many tanks carry 1.6 gpf or 1.28 gpf markings. The 1.28 figure aligns with the EPA WaterSense specification, and it tells you the flush was tuned around a specific water volume — relevant when you are tempted by an "adjustable" universal flapper.

Why the production year changes the part

Manufacturers revise tank internals without renaming the toilet. A model sold for fifteen years may have shipped with two or three different flush valves across those years, and the replacement parts are not interchangeable between revisions. This is the single most common way "I bought the exact part for my model" goes wrong: right model name, wrong production year. When a parts diagram lists a date range next to a part number, believe it. If your tank falls on the boundary, the mechanism in front of you — flapper vs canister vs tower — is the tiebreaker.

Then go to your brand's page

With the stamp read, the model-specific pages translate it into a part:

• Kohler Cimarron / Class Five — canister seal, and why the K-number and year matter for GP1083167.
• Kohler Wellworth and older 2-inch Kohlers — the conventional-flapper Kohlers that universal parts actually fit.
• TOTO G-Max / Power Gravity and TOTO Drake / Drake II — the CST number decides which THU seal you need.
• American Standard Cadet / Champion — three sealing systems; the series number tells you which.
• Glacier Bay / Niagara flapperless — the label number is the only reliable key, and coverage thins on discontinued models.