AFAIK, the 3951 and 3952 modules are required for the 645 AF series. They work on different flashes, so you have to make sure which adapter is required for your particular flash. The 3951 is for the SCA 3000 series flashes. This particular module (for the Pro TL) won't work on the 645 AF, I think in part because it doesn't communicate lens focal length to the flash.
Originally Posted by MattKing