“I think this is great stuff,” said Mitchell Zuckoff, a Boston University professor who published a biography, “Ponzi’s Scheme,” in 2005 and while using material from other McMasters writings had no idea that the publicist’s memoir existed. “My gut tells me he didn’t get his due, but I’m not sure he was a complete Lone Ranger, either,” he said.

Mr. McMasters’s granddaughter and closest living relative, Faith B. Dickerson, a psychologist in Baltimore, said she too had also been unaware of the completed work, though she had copies of various shorter versions. “His role has been shortchanged in Ponzi accounts going back to the 1920s,” she said.

Mr. McMasters was a pre-eminent public relations man when he took on Ponzi as a client in July 1920. A lawyer who had served in the Spanish-American War, he had handled publicity for the campaigns of several Massachusetts political figures, including Calvin Coolidge, John F. Fitzgerald (President John F. Kennedy’s grandfather) and James M. Curley. Mr. McMasters himself ran unsuccessfully for governor in 1938, wrote a play produced on Broadway and exposed a baseball betting scandal.

Ponzi, on the other hand, was already a convicted felon, though Mr. McMasters and the world did not find that out until later. Born in Italy in 1882, he arrived in 1903 and made his way to Montreal, where he served three years for check forgery.

He eventually returned to Boston and devised a novel scheme to build a financial empire based on prepaid coupons that nations issued for postal replies. By buying the coupons at a fixed rate, he could exploit international currency fluctuations by redeeming them at a higher price.

But after offering depositors high interest rates, Ponzi never really dealt in postal coupons, which turned out to be too unwieldy for large-scale speculation. Instead, he just paid off his first depositors with money from later investors who would also have to be repaid. In the end, he was short as much as $10 million  the equivalent of more than $100 million now. He pleaded guilty, was sent to prison, then was deported to Italy and died in Brazil in 1949.