He was a Sahabi Before the conquest of Mecca is the historical event when Mecca was capitulated for and conquered by Muslims on 20 Ramadan, 8 AH.
Hazrat Usman bin Talha (R.A) always kept Ka’aba’s key with him. That’s why he is considered and called as “Sadin of Mecca“.
Since Prophet Muhammad (Sullalahu Alaihi Wassalam) handed over the key to the Kaaba to him, prophet’s companion’s lineage sons have been inheriting it and the title Sadin of the Kaaba until today.