HP21 scientific RPN calculator

The HP-21 was a scientific calculator produced by Hewlett-Packard between 1975 and 1978. It was designed as a replacement for the HP-35, and was the first from so called Second Generation Handheld Calculators. As with most HP calculators then and now, the HP-21 used RPN entry logic, with a four-level stack. It also had a single user-accessible memory register.