If it doesn't have an Enter key, it's not RPN

[d b]

Dave Hicks calls that 1/2 rpn the same thing that Steve S calls it, "adding machine logic". It's was built that way, or maybe that particular way became popular, because accountants and others used to using simple 2 function mechanical adding machines didn't have to change methods. Mark sort-of said that already.

It showed up a lot in the early days of calculators when companies bought a TI chip and built their own calcs around it. They often used that same += & -=, probably to save one key. The Privileg Beginner / Harvard 606, the supercal 616, and the Commodore 6 & 6x all had keys printed +=, -=, x= and /=. All were two level stack RPN, or postfix Only the 6 lacked an enter key but all could be used without ever pressing enter.

Thanks for putting the warning up Mike. Friends don't let friends buy non-rpn calculators.