The HP-35 calculator made by Hewlett-Packard was one of the first calculators to have RPN, as well as a list of functionality that no other calculators had. It was introduced in 1972, and later discontinued in 1975. It was an impressive calculator, boasting advanced features such as a aquare root button, log base 10, natural log, trignometric functions, and so on. This calculator was revolutionary.
More information about the HP-35 calculator is available at http://www.hpmuseum.org/hp35.htm. A javascript based simulator for this HP calculator is available at http://vv.carleton.ca/~neil/hp-35/.