This is a Reinforced Steel Differential Crossmember for Nissan R180 or R200 equipped 240Z 260Z and 280Z.
This crossmember allows the use of the aluminum finned large capacity differential covers from ESCORT. As can be seen in the photos, the expanded area clears the crossmember perfectly.
ESCORT recommends these for cars with up to 250HP. Due to the suspension geometry, large amounts of power encourages the rear control arms to want to move laterally away from the differential. This does a sufficient job of counteracting that type of movement but at higher output, ESCORT recommends their machined aluminum differential crossmember.