“LET’S start with Act IV, No. 51,” Leon Botstein, the conductor, says after bringing the rehearsal to order. “The happy ending.”

He lifts his hand, and mournful violin notes begin, signaling Romeo’s arrival at Juliet’s grave and his glimpse of her seemingly lifeless body. But soon a slow pulsing of harp and strings is heard — inhalations and exhalations — as Juliet starts to breathe. A melody that accompanied her first appearance, carried by the glockenspiel and flute, returns, followed by the silvery sound of tinkling bells and shimmering chimes that land like a sprinkle of fairy dust. The pace then quickens in a joyful scherzo with clicking castanets and plucked strings, as Romeo and Juliet embrace and go off together.

A happy ending, for real.

The members of the American Symphony Orchestra who were in the south hall of Riverside Church on a recent crisp autumn afternoon had gathered not only for a rehearsal but also for a premiere of sorts. The score — the original version of “Romeo and Juliet” that Sergei Prokofiev wrote in 1935, just as Stalin’s Great Purge was gaining momentum — had never before been performed.

“It’s a great thrill,” Erica Kiesewetter, the concertmaster of the American Symphony and a member of the American Ballet Theater orchestra, said during a break. The heady buzz of revealing an unknown masterwork has infected everyone, she said.