rockwell 330
this is a very natty desk calculator. it displays and
calculates to 14 digits. see here sqrt(99).
it functions like most desk calculators, where the plus button is
predominant and = is only used for * and /
there are two memories I and II. when used small annunciators appear on
the left of the display.
the left side of the keyboard offers a generous array of memory
operations. you can add or subtract the current number to memory within an
operation (M+ and M)
or you can complete the operation and add or subtract the result from
memory (=+ and =) 

also a switch offers a tally mode into either memory1 or 2 similar to other
such machines. rather than having a digit switch for 2, 3 or 4 decimal places,
you can set this number with the DEC SET button and a switch is used to specify
the rounding mode. 