A tang stamp is the maker’s mark pressed or etched into the tang of a blade, usually visible on a folder when the knife is open. On traditional American brands it carries the name, the country of manufacture and often a code that dates the knife.
At minimum, a brand name. Beyond that, tang stamps commonly carry a country of origin, a pattern number, and for some brands a dating code. One example is Case: from 1970 the company placed ten dots under the CASE XX USA stamp and removed one per year, so nine dots is 1971 and one dot is 1979, with a different system taking over for the 1980s.
Buck’s markings changed over time rather than following one scheme: a name-only stamp before 1968, a two-line BUCK / USA mark from 1968 to 1972, a three-line mark with model number from 1973 to 1981, and a formal date-code symbol system introduced in 1986.
The stamp is the closest thing a production knife has to a birth certificate, which is why two visually identical knives can be worth very different amounts. It is also the first thing to check when a listing claims an age.
Be careful with decoder charts found online. Only the eras and milestones a brand documents itself are reliable. If a chart claims a precise year the manufacturer has never published, treat it as folklore.