Heartfield's final drive unit is a Dana, which is widely considered inferior equipment; most other XJ's have the better Salisbury/GKN unit.  One difference is apparent in those bolts that hold the brake caliper to the differential housing; on this Dana unit, the bolts are installed from the outboard inward, requiring a hole in the disc for access to the bolt (the TWR doesn't have such a hole, though, so you must separate the outer disc from its hub to disassemble).  On the Salisbury/GKN unit, the caliper mounting bolts are installed from the inboard side outward.

This second picture seems to show a "step" in the face of the outer disc, but Heartfield says this is an illusion; the outboard surface of the outer disc is totally flat, and that circle visible is merely a light polishing from turning the rotor around during fitting.

Heartfield's calipers are the standard item, except that they have been reassembled by TWR with a 1/4" spacer between the halves.  This same method is used by most doing such a upgrade, since it simplifies the modifications and ensures that the brake effectiveness is not altered (which might screw up the front/rear balance).

Heartfield himself is responsible for the green paint.

This next picture was taken several thousand miles later.

Heartfield's vented rotor is 0.78" thick -- effectively 3/4", or 1/4" thicker than the baseline solid rotors.  The hub, which is called a "hat" in the brake biz because that's what they look like on most cars, is in fact a flat disc 3/8" thick.  Since the "height" (distance from the inboard friction surface to the flat outboard surface of the hub, or the height of the center when the rotor is set on a table) of the solid rotor is 1", the TWR arrangement effectively adds about 1/8" to each side of the solid rotor location.