I have had this since about July 2003. As far as I can make out it is a very rare piece of Beeb equipment. A host adapter allows you to connect an appropriate hard disc drive to your BBC. It performs all the relevant interfacing between the BBC 1 MHz Bus and the hard disc controller.
I bought it when I had my FileStore crisis when I thought I had broken the host adapter in my FileStore E20. Thankfully, all I had done was to melt the ribbon cable so I have a spare host adapter.
I have tried to attach several hard discs to it but without success. I think the drives I am using are just too darn modern.