I was aware of this, but didn't realize that the module stays with the camera. I'll keep a lookout for a Nikon module. Thanks for the info.
I should have mentioned-the module comes with a metal piece with a cold shoe on it. It's for mounting the module onto the part of the 555's bracket that is fastened to the camera, if you don't have a shoe elsewhere. It comes with a thumbscrew to tighten it. It fits in the slot on top of the camera bracket.