Какова была главная особенность пис

Какова была главная особенность письма, которая помогла установить её дату?

Результаты (английский) 1:

What was the main feature of the letter, which helped install her date?

Результаты (английский) 2:

What was the main feature of the letter, which helped establish her date?

Результаты (английский) 3:

What was the main feature letters, which has helped to establish the date?