PUNE, May 1 -- On February 18, 1665, British representative Humphrey Cooke walked across the rocky stretches of Bombay island and formally asserted British sovereignty over the territory in a symbolic ceremony that would alter the future of western India forever.

More than 360 years later, as Maharashtra celebrates its 66th anniversary of its formation on Friday, a Pune-based researcher uncovered a rare Portuguese manuscript in Lisbon last month that reconstructs this defining moment in the birth of modern Mumbai.

The 10-page Portuguese document, titled 'Instrument of Possession', was traced by researcher Yashodhan Joshi in the archives of DigitArq Portugal - the digital repository of the Portuguese National Archives that preserves cent...