The American economy was in recession, and the Dow Jones Industrial Average was yo-yo-ing in the 600s when Marjorie and Malcolm Gray found a new investment to offset a battering in the stock market. It was the spring of 1960, and after dropping by a neighbor’s home on Long Island to hear a couple of hopeful musicians play songs from their new Off Broadway musical, “The Fantasticks,” the Grays bought a stake in the show for $330. The other day, 50 years later, Ms. Gray. now a widow, received her latest dividend from the musical, a $200 check in return for that stake. She has earned about $80,000 so far on the original $330 investment, or an average of $1,600 a year since 1960. And she will continue to receive money until 2020 because of the investment terms for the original production, which ran a record-setting 42 years at the Sullivan Street Playhouse in Greenwich Village. Investors or their heirs will make money from all “Fantasticks” shows — including a current revival Off Broadway and a forthcoming run in London — for 18 years following the final performance of the original show, which took place in 2002. The 18-year period is not unusual (though 10 years is more typical today). What has turned “The Fantasticks” into an extraordinary legacy for the investors, many of whom are now in their 80s and 90s, was the longevity of the 1960 production, the 50th anniversary of which will be celebrated by the surviving investors on Monday.