V&A curators who puzzled for decades over what might be inside a vintage Christmas cracker used an x-ray to discover it contained a small chocolate – but since it was made in the 1920s none of the staff were tempted to sample it.

The cracker, which has gone on display as part of the Staging Christmas exhibition in the museum’s theatre and performance gallery, was already more than half a century old when it came to the museum in 1987 with a small collection from the estate of the actor Myrette Morven.

Born in Dublin, she was a chorus girl in many hit musicals in the 1920s – including Rose-Marie, billed as “A Romance of the Canadian Rockies”, which opened at the Theatre Royal in Drury Lane in March 1927 and ran for 581 performances.

An x-ray of the cracker, showing its contents. Photograph: Victoria and Albert Museum, London

Morven was 19 when she featured in one of the highlights of the musical, a totem pole dance in the first act.

One critic wrote: “A wonderful piece of concerted work – half ballet and half military drill, with amazing and dazzling effects that leave one breathless. When all these girls – literally dozens of them – fall down like a crop of brilliant tulips falling before a reaper, the effect is extraordinary and there is nothing left to do but cheer.”

He noted that the cheers went on for so long that the principal comedian had to come out to tell the audience that the girls were scrambling into their costumes for the next scene and could not take another curtain call.

Souvenirs from the show included dolls and wooden figurines, as well as Tom Smith crackers decorated like a totem pole. Morven – who never became a major star but had a successful career on stage and screen – presumably bought or was given a box of the crackers, and cherished one unopened one for the rest of her life. Its secrets are only now revealed, 30 years after her death.

