I don't think the 'M24' has any relation to the turbo, it's just something that's casted onto the compressor housing. It may relate to the type of compressor housing flanges for the inlet and outlet i.e. bolt on or hose type.
every garrett turbo has a plate with one mount point on the actual core of the turbo... if it hasnt got the plate i would b thinking that it is not a garrett turbo
without that little plate dude its next to impossible to tell u what it is without the measured sizes of the wheels..... but its not a stock turbo off any silvia or skyline....
looks like an older model t28 at best.... and if i were going to use it, it would b getting rebuilt b4 it went near my engine
anyways ..!!
that turbo is very old , i wouldnt touch it either .....i can connect you to a guy who is selling a stock turbo from an R33 skyline for quite a good price !