This Anita 831 is just a small incremental step from the earlier Anita 810 and 811 calculators, with an added square root function the most obvious addition. It is styled in the same manner as the 811 and retains 3 switches, the left most two of which have the same functionality as the 811 (power and rounding to 2 places). The memory switch has gone, and is replaced with a circed '2'. This places the calculator into automatic 2 decimal point mode. With this switch activated, typing the number keys will 'left shift' them through a forced two decimal places, allowing accounts to be totalled more easily. At any time it can be overridden by pressing the decimal place button, when the decimal place snaps back to the rightmost position. I don't know of any other calculator with this feature, but I can see why it hasn't really persisted.
Less obvious enhancements include the dispensing of the 'reverse polish' style operation of the 811 '+' and '-' functions, and it acts more like a modern calculator. Also the memory functions are more sophisticated where a displayed number can be added or subtracted to the memory by pressing the 'm' key followed by the '+' or '-' keys respectively. To recall the memory value, 'm' followed by '=' is entered. The display is also slightly different from the Anita 810 and 811 calculators, with more widely separated numbers, as is also true of the later Anita 841 model.
This particular device has a PCB date stamp internally of the 28 Nov 1974. Interestingly, the Rockwell I.C. date code is 7452 (week 52, 1974), which post dates the PCB stamp. I don't know why this could be, except to guess that the PCB should read 28 Dec, though I can't remember the factory being open over the Christmas week.
Switch it on---it works! There's no manual for the 831 on this site, but one may be found on Nigel Tout's Anita site.
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