One of the world’s first digital computers to replace the handwritten calculations of human “computors” is getting an official reboot that could lead to a spot in the Guinness Book of Records. The 61-year-old Harwell Dekatron—about the size and weight of an SUV—was originally hailed as a slow, steady machine capable of delivering error-free calculations while running for 90 hours a week. It has survived to become the oldest original working digital computer following the announcement of its completed restoration by The National Museum of Computing in the U.K. on Tuesday. “In 1951, the Harwell Dekatron was one of perhaps...