However, doubts persist because the dark hair in the painting failed to tally with Hiffernan's red locks.
The mystery appears to have been finally solved by Claude Schopp, the French historian and biographer, while he was combing through correspondence between the French authors Alexandre Dumas fils — whose father wrote The Three Musketeers — and George Sand.
One passage, in which Dumas fils was criticising Courbet, flummoxed him as it read: "One does not paint the most delicate and the most sonorous interview of Miss Queniault of the Opera."
Taking a closer look at the handwritten original, Schopp realised the word "interview" had been mistranscribed. In fact, Dumas fils had written "interior".
"It was a thunderbolt. Usually I make discoveries after working away for ages," said the writer, whose book on the find will be published this week. "Here I made it straight away.
It almost feels unjust," Schopp told AFP.
The biographer showed his discovery to Sylvie Aubenas, the head of the French National Library's prints department, who said: "This testimony from the time leads me to believe with 99 per cent certainty that Courbet's model was Constance Queniaux."
- Telegraph Group Ltd