Matthew Rothman bought an HP 12c financial calculator for his first job out of college in 1989. Years later, he still has the same calculator. And he still uses it constantly, just like thousands of other 12c enthusiasts. “Whenever I switch jobs, I just peel the old business card that is on the back and tape my newest one on,” says Mr. Rothman, head of quantitative equity strategies at Barclays Capital in New York. Sales of the device, which debuted in 1981, haven’t slipped even after its manufacturer, Hewlett-Packard Co., introduced more-advanced devices or even, two years ago, a 12c iPhone application, which replicates all the calculator’s functions, the company says.