The climax scene in the 1975 film, little known despite its stars being reigning icons of the time, gave restorers a glimpse into the magnificence of Mumbai's opera house, which reopened last year after seven years of painstaking work.
A listed heritage building in Baroque style, the Opera House was inaugurated by British monarch King George V in 1911 and completed in 1916. It began its journey as 'the finest theatre in the East', but suffered monumental neglect during its stint as a cinema hall before its eventual closure and abandonment.
Mumbai-based conservation architect Abha Narain Lambah, who led the restoration project for the former royal family of Gondal, the current owner, says it was "leap of faith" given the myriad challenges they faced.
"It was an old film starring Rajesh Khanna, Shashi Kapoor and Mumtaz that helped us in knowing vital components of the old design. A scene towards the end of the movie was shot in the Opera House, where the original balconies and the frescoes could be seen. It was a big help," Lambah told PTI over phone from Paris.
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"We looked at some of the old documents and pictures we could manage for reference," Lambah, who also helped restore Crawford Market and Bombay University's Convocation Hall, said.
Over the decades, the place assumed different avatars, from a regal theatre that attracted the creme de la creme, to a decrepit single-screen theatre that was eventually abandoned.
"Prem Kahani", a romance set in the 1940s, provided a glimpse into what once was through a magic show in the theatre.
The Opera House also figures in the 1948 film "Aag", directed by Prithviraj Kapoor's son Raj Kapoor, who also played the lead role.
And, taking the Kapoor connect further, the younger Raj Kapoor was played by his younger brother Shashi Kapoor.
Earlier this month, UNESCO honoured the building with the 'Award of Merit' in the 2017 Asia-Pacific Awards for Cultural Heritage Conservation.
"Under generous private patronage, the heroic rescue of the Royal Bombay Opera House from the verge of collapse has revived the beloved cultural venue as a gift to the city of Mumbai," the jury said in the citation.
It has been restored under the guidance of dedicated specialists in overcoming severe structural distress, restoring decorative features and improving acoustics, all the while maintaining spatial integrity and authenticity, it said.
In its heyday, the entertainment venue rivalled the best in Europe.
Now, the Opera House has become the toast of Mumbai again as a venue for concerts - pop, classical and jazz - plays and films, scripting its own story of revival and restoration.