Period graphics are researched by reconstructing the visual grammar of a specific time and place - the typefaces, printing methods, paper stocks, administrative conventions and language in use - so a prop belongs to its year rather than merely looking old.
What you check
The type in use and how it was set; the printing method and its characteristic wear; the paper stock and how it ages; the administrative, legal and currency conventions of the period; and the language itself, including spelling and orthographic rules that have since changed.
Common pitfalls
Anachronistic fonts are the usual tell. For Greek work, capitals drop the tonos (ΕΛΛΑΣ, never ΕΛΛΆΣ); dates and currency follow the period; a form from the 1960s should not be set in a typeface that did not exist yet. These are the details only a trained eye catches - and exactly the ones that break the illusion.
How it works
Each era is reconstructed from references used for grammar and structure - the plate shape, the type hierarchy, where a border turns a corner - never by copying a real mark. Everything produced is fictional by design. Recurring research is captured in Chronotype, which sets paperwork in English, Greek and German and remaps type to Greek-capable faces where a Latin display font has no Greek.
Why it matters
A prop reads as real or it does not, and the tell is almost always the type and the era. Getting the invisible details right is what lets a graphic object disappear into its world.
Related: What does a set graphic designer do? · What are graphic props for film?