This kit is a set of weld-on steel brackets, designed to give the 200SX/S12 and 300ZX/Z31 an extra caliper attachment point on the trailing arms, most likely for a hydraulic handbrake, though other uses are possible. The kit includes 2 steel caliper brackets, and 4 JIS flange head bolts. The kit may require some grinding to fit perfectly, and of course welding. Note that if the main body of the bracket is welded properly on both sides, the thin legs can be trimmed or cut off entirely; they are provided mainly as a guide that holds the bracket in place for welding.
Also, being on the leading edge of the hub, the motion ratio is smaller, so the caliper (and bracket) affects unsprung weight much less than being on the rear. Therefore adding the second caliper should not adversely affect handling, especially one of the more modern aluminum calipers from a Skyline or 1990’s 300ZX.
Keep in mind that mixing and matching calipers is not always possible directly. For example, the 1983-1986 300ZX and 200SX come with unvented/solid rear rotors, they have a slimmer profile trailing arm/hub assembly than the 1987-1989 300ZX; bolting on a vented late-300ZX rotor would be difficult, and therefore using a Skyline caliper for vented rotors would be difficult.
Late-300ZX trailing arms are a direct swap into 200SX V6 models with the IRS, which is probably the easiest option if available, then the Z31 rear caliper can be mated to the S12 parking brake cable in the rear, and a Z32 caliper can be installed on the new bracket. The hydraulic line can be moved from the Z31 caliper to the Z32 caliper for additional clamping torque, while the parking brake via cable can be retained on the Z31 caliper.
To use a 300ZX/Z32 or Skyline/R32 rear caliper with the early-300ZX or any year S12 trailing arms will likely require machining a rotor spacer, and maybe re-drilling the rotor to 4-lug. Then either a ’87+ Z31 caliper or a second Z32/R32 caliper will be required to fill the duties of the pedal brake.
Though having an entirely hydraulic handbrake can be less reliable than a mechanical one, and may be illegal in some jurisdictions. Conversely, most of the racing/drifting regulating bodies have banned pass-through hydraulic handbrakes, and require fully independent hydraulics to pass inspection.