This is the Naturescapes GSP-35 Safety Plate for Gitzo Tripods. It is meant to be used under the tripod's mounting plate, between it and the tripod head. The aluminum plate provides extra support and prevents twisting and loosening which might result in damage to valuable equipment.
The GSP-35 is designed for use with Gitzo's 2008 or prior Series 3 models; for example: GT3530S, GT3530LS, GT3541LS, GT3541XLS etc. Basically, it is compatible with any tripod that was made before 1998 in this series that employs Gitzo's SafeLock plate. The GSP-35 is also for use with Gitzo's older tripod models that begin with 13xx, such as the GT-1325, GT1348, etc.
The plate is not compatible and not needed for the latest 2012 Gitzo models such as the 3532LS etc.