Many thanks for the replies, I did wonder if there should be a centre cone. I'll try and get a better picture of the internal details of the body.
Parts of it look home made, for instance the threads are rougher than would be expected on a professionally made item and it seems to have spent some time half submerged in water. The outer body looks to have been chrome plated but was damaged by vice jaws so I have machined it smooth.
I think it will do any milling that I might be doing.