I even added an early LCD screen to mine for a very easy to carry machine. To study how exactly the infix notation and postfix notation differ we used on of the very old HP calculators. Infix to postfix was common to computer scientists in 1970. But this was back when very few colleges even had computer science programs for undergraduates. Postfix notation was not common to average people who use the infix written system.
Computer scientists tend to find postfix to be more 'pure' than infix.
It does have the qualities of left to right operation and no parentheses. But it requires a stack, which would translate to extra memory steps for a human if we wrote expressions in postfix. The computer scientist view of postfix is similar to the scientists' view of the metric system. At Hewlett Packard we were so proud that our calculators, the first scientific ones ever, were years ahead of competition. They used postfix partly because the least logic or ROM chips were quite expensive back then.