I was working with my HP-41 and my HP-28 side by side to compare results. I suddenly noticed something that as a relative newcomer to the HP product line I had not noticed before. The HP-41 has the arithmetical operations keys to the left of the numerical keyboard while the HP-28 has the arithmetical operations keys to the right of the numerical keyboard. Furthermore, reading the operations keys from the bottom up on the HP-41 the sequence is / x + - while for the HP-28 the sequence is - + x / . The layout on the HP-41 seems to have been used on every HP handheld from the HP-35 through the HP-41 with the exception of the HP-10. The layout on the HP-28, which is the same as that on the TI's and most other calculators, seems to have started with the Voyagers and persisted in the HP product line ever since. Back in the HP-35 days the desktops such as the HP-91 and HP-97 had the HP-28 layout.
Why did HP decide to place the operation keys to the left of the numeric keypad on handheld calculators from the HP-35 through the HP-41?
Why was the change made with the Voyagers and beyond?