the 105mm nikkor leaf has its own helicoid - the lens is incompatible with the normal helicoid.
the thread that you have discovered is the 57mm x 1 thread. it is primarily, but not exclusively for the mounting of the standard macro rings. users for years have successfully mounted all sorts of non-bronica lens to this camera because of this 57mm x 1 thread. none of the standard bronica lens - including the old nikon rangerfinder lens that could be fitted on, use this thread. the bayonet mount on the inside of the shutter speed ring is for the old flash-sidebar attachment. they occasionally turn up on eee-bay.

hope this helps