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

[Derek W]

The entry system using += and -= was quite common on early calculators. My first calculator (in 1974) was a model similar to that, sold as Commodore in UK, with a switchable constant but no memory keys. It cost me GBP30 at a time when my annual salary as a student engineer was GBP832, so maybe equal to GBP 500-600 or USD800-900 today. A mains adaptor was an extra GBP5.

It was a whole lot easier than a slide rule for approximate calculations or 7-figure log tables for accurate ones!