Waheguru
This janamsakhi called 'Sakhi Mehl Pehle Ki' written between 1570-1574 by Seehan Upal. Seems to be the oldest biography of Guru Nanak Sahib Ji. Bhai Seehan Upal was a important sikh mentioned many times in early sikh literature and who personally knew the first three Gurus.
If anyone has any more information on this janam sakhi please share
Sakhi Mehl Pehle Ki By Seehan Upal