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. A shift key provided access to functions whose legends were printed on the fronts of the tall trapezoidal keys.

To enhance practical usability display was modified to show both X and Y register. Also I have eliminated mechanical DEG/RAD switch and moved the function under DSP button.
HP21 has few big enough buttons to be used comfortably on mobile devices.