4 Functions, RPN
|"This is an unusual, and
somewhat rare machine. My guess as to the date of this machine is sometime around 1973 or
1974. This calculator was given to me as a birthday gift by my parents. I was one of the
first kids in school to have a calculator, and it definitely raised some eyebrows with the
teachers, who hadn't had much experience with a student having a calculator.
The Sperry Remington 661D is unusual in that it has only a 6-digit display, with a special key on the keyboard that displays the remaining digits of any result that is larger than 6 digits when pressed and held. I have seen this arrangement on some early Casio machines, leading me to believe that Casio used the same 'CPU', or perhaps one vendor OEM'd the calculator from another, and just made packaging changes."
Source: Rick Bensene's Old Calculators
X-Number World of Calculators