This QR-XL plate is a long plate with 2 slots for custom mounting of cameras to the PCH or CBH-6 heads.

Its use is dual purpose. It allows for setting the center of gravity above the center of the gear head or ball head for perfect balance, especially with longer and larger lenses.

Secondly, this plate allows you to find the ideal (nodal point) center of the lens to be placed above the rotation point for spherical  or round-shot captures.