This just beat me to it. Here's my take on the earliest personal, portable computing devices:
Slide rules fully deserve their place in this history.
Several generations of engineers and scientists used slide rules, an essential tool from their invention in the early 1620s that remained in common use into the late 1960s. Arguably, these were the first portable calculation devices unless you want to include the far earlier, but less capable, abacus which, in its modern form, was in use from the 2nd century BC and printed tables of logarithms, which appeared in 1617.
However, a slide rule, though portable and deserving its place in every engineers' desk or briefcase, wasn't really pocketable. The Curta calculator gets that slot. It was far from the first mechanical calculator, but all its predecessors were heavy, bulky desktop machines, while the Curta was a comfortable handheld device and much loved by the rally-driving fraternity.
This was the first really good pocket electronic calculator. It was not only affordable, but a real game changer because it was the first that could replace a good slide rule, being capable of dealing with logarithms and trigonometric functions.