Sikh tradition holds that in 1604 Guru Arjan, the fifth Guru, gathered the compositions of his predecessors together with those of his own and of a number of Hindu and Muslim devotional poets, dictated them to Bhai Gurdas at Ramsar near Amritsar, and installed the completed volume in the Harmandir Sahib on 1 September of that year, with Baba Buddha as its first granthi. That volume is the Adi Granth, the first Granth, and everything in Sikh scripture descends from it. When Guru Gobind Singh conferred the status of Guru on the scripture itself in 1708, it was this text, with the ninth Guru's compositions added, that became the Guru Granth Sahib.
The volume kept by the Sodhi family at Kartarpur, near Jalandhar, is held to be that manuscript. The Sodhis are descendants of the Guru's line and have held it privately for generations. The Shiromani Gurdwara Parbandhak Committee, which administers the historic gurdwaras, has sought custody more than once and the family has not agreed.
If the identification is right, this is the most important physical object in Sikhism: not a copy of a scripture but the scripture, in the form in which it was first fixed, from the hand of the man the Guru dictated it to.