From Z1's Web Site:
You may choose between the Standard Non-Thermostatic Mocal Sandwich Plate adapter and a Thermostatic model. It is highly recommended that a Thermostatic sandwich plate adapter be purchased when installing an oil cooler onto a street driven 370Z. As important as it is to keep high oil temperatures under control, it is just as important to maintain the proper temperature to ensure the highest possible lubrication.
Z1 Motorsports 370Z Oil Cooler Kit
Basically, the Thermostatic Plate Adapter allows you to use a thermostat to bypass the oil cooler in winter months on the street so that your oil can heat up to the proper operating temperature.