Yes the normal 4x5 holders will definately not fit.
I dont have the camera in fromt of me right now. but I think there is a channel that the holders should be slid into. There is also a brass 'tooth' shaped locking mechanism.
The back is made to be reversible. It comes off easily by lifting a spring over the pins. So I will probably make a replacement 4x5 back if I cannot find a useable holder.

It's too bad there is not a makers mark on the camera. They are really ingenious when you look at how they are engineered.