this model is internally very similar to the fx-39.
indeed it is almost the same calculator except for a ten digit display and
a slight button rearrangement.rather than having the mode switch on the side, the 120 has two on the
front panel. one for * deg/rad/grad* and one for stats mode. this means that
the trig mode is selectable whilst using stats. also the *inv* button is moved
from the left to the right up next to the switch. the left button is now
percent on a key of its own.

you might think that nCr and nPr along with polar to rectangular
conversion have been added in this model, however, the fx-39
also implements them on the same keys except that this is not indicated on the
front panel nor mentioned in the manual. i dont know why if it did indeed
work?

the ten digits of display is certainly an improvement, but the accuracy
is the same as the 39. trigs and logs only display 8 significant digits.
however the 10 digits benefit basic calculation, square root and fractions
which, with the casio fraction spacer, really need 10 digits of workspace.

this model is a bit rarer than the fx-29 and fx-39 because it didnt
really do much more and came out later, when there were other models
available.

when i acquired this machine, it suffered from the notorious sticky
button problem that often affected this and similar models. see here
how to dismantle and repair this.