Elektronika MK-61 Wikipedia
The Elektronika MK-61 (Russian: Электро́ника МК-61) is a third-generation non-BASIC, RPN programmable calculator which was manufactured in the Soviet Union during the years 1983 to 1994. Its original selling price was 85 rubles.
The functionality of the MK-61 is identical to that of the MK-52, except the MK-52 has an internal non-volatile EEPROM memory module, for permanent data storage and also has the capability of using external EEPROM modules. The MK-61 is functionally very similar to the MK-54, the B3-34 and B3-21 Elektronika calculators, all of which are renowned for having a very large number of undocumented functions.
The MK-61 has 105 steps of volatile program memory and 15 memory registers. It functions using either three AA-size battery cells or a wall plug. It has a ten-digit (eight digit mantissa, two digit exponent) green vacuum fluorescent display.