This is the replacement Mamiya Terminal Adapter RA401 which connects the Mamiya 645's circuitry to the Pro Left Hand Grip GL401 (B&H #MAGLH645P). The RA401 adapter connects from the base of the grip to the electronic cable release terminal and finally in to the "PC" receptacle.
Note! Mamiya makes two Left Hand Grips for the 645 Pro series: The Left Hand Grip GL401 which is for the 645 Pro model, and the Left Hand Grip GL402 which is specifically for the 645 Pro TL model. If you currently own a GL401 Grip and upgrade your camera to a 645 Pro TL model, you only need to purchase new Terminal Adapter RA402.