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

[Michael de Estrada]

I recently bought a vintage calculator (JCPenney MM3R) under the mistaken assumption that it was a non-HP RPN. It lacked any sort of RPN indication such as a key labelled Enter or Ent or <up arrow> or even Input, although it did have keys labelled += and -= . A generally reputable eBay seller claimed it was RPN, however, that proved not to be the case. Apparently, the seller had only checked addition and subtraction, which are entered the same for simple operations, however, they are different for multiplication and division. So, 3 Enter 4 + and 3 += 4 += yield the same result of 7, but 3 Enter 4 X yields 12 whereas 3 += 4 X yields no result until += is pressed at which point the result is 16 ! So, basically, if you see an auction for a calculator w/o an Enter key claiming to be RPN, ask the seller to test multiplication to verify that this is true. Every single one of my many non-HP RPN calculators have some sort of Enter key on them, although I do know of a few exceptions such as the National Semiconductor NS600.

Note, I bought this calculator from a different seller than the one claiming it was RPN. This seller made no such claim.