That's a standard TA06 gear diff (in black rather than white) with steel internal gears included which used to be an option (#54428). The casing is certainly not steel...

Edit: I looked at that picture again and there are two different assembled diffs... one does look like it could be even made out of steel but that would be madness, what's going on in the pic?

Well the TA06 (and TRF511 w/Upgrade Kit that I'm running) both use the same diff and yes, it's 39t. Whether it's compatible with the M chassis I wouldn't know (it's pretty big!).