Tuesday marked the 50th anniversary of the 1962 armed robbery of a mail truck carrying $1.5 million in cash. The truck was traveling north through Plymouth on Route 3, carrying deposits from banks on Cape Cod to the Federal Reserve Bank in Boston. Five miles north of the Clark Road exit, two cars blocked the highway, forcing the mail truck to stop. The driver and guard were disarmed, tied up, and left in the back of the mail truck, according to an account written by Gail Sullivan Begley and included in a local history book, “Beyond Plymouth Rock, America’s Hometown in the 20th Century.” The loot was never recovered and the robbers — authorities believed there were six of them — were never caught. Plymouth Public Library is planning an event commemorating the anniversary in October.