PHILADELPHIA — In 1921, the wily art collector Albert C. Barnes wrote to Paris to his friend and fellow collector Leo Stein, who was in dire need of money and had deputized Barnes to sell some of his holdings in the United States. They included five watercolor landscapes by Paul Cézanne, but Barnes reported that he had failed to find “anybody who seems to think they are sufficiently important to want to own them.”

It was pure mercantile flimflam. Barnes turned around and bought the watercolors for himself, at $100 each, installing them permanently in his personal museum near here. Now it turns out that Barnes got a better deal than even he had thought: A conservation treatment of the watercolors has revealed two previously unknown Cézanne works — a graphite drawing and a watercolor with graphite — on the verso (the reverse side) of two of the watercolors.

Hidden beneath brown paper backing, the newly discovered pieces are unfinished, but they have sent tremors through the world of Cézanne scholarship, where additions to his body of work are exceedingly rare and where even the resurfacing of long-unseen pieces can be huge news. A watercolor study for Cézanne’s coveted “The Card Players” paintings, discovered in Dallas in 2012 after a six-decade absence from public view, brought $19 million at auction that year, an indication of the work’s importance but also of Cézanne’s place among the most sought-after artists of the 19th and 20th centuries.