well my T38 is made up from;

Escos based T34 (with the .63 exhaust housing)
and the housings have been machined to accept the wheels from a T04B (the RS500 T4 is a T04E )

oh and mine's a Universal Turbos version,a few of the other companies make slightly different variants which come with the compressor housing similar to a T4.

i hope that helps a little,if you have any more questions just ask