HP-45 advanced scientific pocket calculator, 1973
The HP-45 was HP's second scientific handheld electronic calculator. It added many features to the HP-35, including numbered data registers, store and recall register arithmetic, built-in conversion constants and selection of trigonometric modes.
The HP-45 contained a hidden timer. Originally, the designers intended to include a timer, but they ended up leaving out the quartz oscillator which was necessary to make the timer accurate. Therefore, while the timer software was left built-in, the timer was not accurate, and was never exposed as a user feature.
The HP-45 was HP's third handheld electronic calculator and the second scientific model.