according to the charts on the NisTune site (here ) , that is the ECU out of a R33

looking on the Nissan Part list one, it shows the 21Uxx numbers belong to the r33 rb25, as well as the MEC-Rxx numbers belong to rb25's

then you look at the nistune board match list and the 21Uxx numbers belong to the r33 rb25 and it does imply that they are interchangeable.

given the numbers on it, it just looks like it was sent to nissan at some point in its life and was re-manufactured.
the numbers won't tell you if it has been modified.
Usually, it will be really obvious if you take the covers off. you'll see the chip in there. (that isn't stock) Most tuning companies will have their logo on the chip or something inside the cover to tell you who's it is.
If it isn't obvious, then most likely it isn't modified.