Could someone identify the blue part (marked on the board SC81EIA4R7M), please?
It's used in an old calculator Casio FX-602P, manufactured in year 1984. The part is connected across the power supply pins of the static RAM chip uPD444G. It could be a capacitor (but other capacitors on the board look ordinary and are marked differently), or perhaps a varistor or a Zener diode.
Here's the FX-602P circuit diagram drawn by me: