The clips for the 645 Pro and Super are entirely proprietary - there is no 3rd party substitute that will work (including Optech USA). The problem with 3rd party straps is that the connection clips interfere with winding (either the manual winder, or the power winder) on the right side of the camera.

If you have the clips, I believe that you can feed 3rd part strap material (of the right size) through them, but I haven't tried it.

Matt