I used a TC3 nut. And it is much like the old T3 (and prior) Associated diff nut.
Went together like butter on hot toast.
I don't fault Ass. for much (every time I leave and come back to this hobby, it's what I choose to drive), but they certainly @#%$ up by not taking a couple of seconds to throw the right length bolt in the TC5R and/or TC5F kits(or anything going from the plastic outdrives to the "team" pieces).