I recently purchased two calculators from: * Casio fx-115D SUPER-FX and * Radio Shack EC-4004 (I'll talk about this in another post). Date the fx-115D SUPER FX was first manufactured: about 1995, according to calculator.org ( I honestly thought this model was around in the late 1980s or early 1990s. Hard to tell when Casio does not date their user manuals. As with most solar-powered calculators, the fx-115D SUPER FX (herein referred to as fx-115D from this point forward) has a battery backup. This means we can store data and it will be retained after the calculator is turned off. In addition to the independent memory (M), the fx-115D has six additional memory registers. The Kin key stores the number into one of the six registers, which the Kout key is the recall key.