Making the job particularly complex, Ms. Belloli had more to worry about than just the image of “The Actor.” There was a painting on the back of the canvas, and that had to be considered too.

For years only a few scholars knew that this second painting existed, and they debated among themselves what it could be. Was it a discarded work by Picasso or, as John Richardson, Picasso’s biographer, hoped, the missing 1901 painting titled “Virgin With Golden Hair”? Hubert von Sonnenburg, a former conservator at the Met, hypothesized that perhaps it was a stage decoration by someone else because the canvas was thick, not the fine artist’s weave a painter would normally use.

“The canvas and composition do suggest a work intended as a decoration,” Mr. Tinterow said. “And the colors were particularly theatrical and vivid.”

“The Actor,” made in the winter of 1904-5, dates from a period in Picasso’s life when he was particularly poor, and he often employed whatever canvases or materials he could get his hands on even if they had already been used. He may have tried to obliterate the original composition by painting over it, but X-rays taken at the museum revealed a landscape with stones in a rippled body of water, rocky palisades and a large figure that might have been a female nude, although Mr. Tinterow said it was impossible to determine that with certainty.

The bold, swirling brushstrokes and palette were definitely not Picasso’s. Rather the colors — gold, mauve and cerulean blue — were in keeping with the work of Symbolist painters in Barcelona who appear in caricatures by Picasso. “It could have been done by Isidre Nonell, one of the Symbolist painters who had a studio in Paris and was known to have given Picasso materials in 1901,” Mr. Tinterow said.

Whoever the artist was, the X-rays showed that the landscape was painted horizontally, and that Picasso rotated the canvas for the vertical composition of “The Actor.”