This is the Xebec Winchester disc controller board.
In the days before SCSI discs, hard discs had a separate controller board connected to them, and this is such a board.
Companies like Seagate and Rodime and Western Digital and NEC all made Winchester disc drives which could only be used if they were attached to a controller board like this one.
I was very interested in getting this item when I first saw it as I knew that the Xebec controller board was known to work with the BBC micro. Suitable controller boards are very hard to get hold of in my experience, whereas I have an abundance of ST506 type Winchesters waiting for a flood of controllers to come along.
This is the Xebec S1410 Winchester controller board and it is very similar to the Adaptec controller which is more common in BBC applications. On the left it has a power socket at the top and a 34-way edge connector and two 20-pin headers to enable up to two Winchester discs to be connected. On the right hand side is a 50-way header for attaching to a host adapter board, and a bank of jumper links.
The big difference between this controller and the Adaptec ACB4000 series controllers is that the Xebec one does not store the drive shape on the disc to be read into the controller every time it is reset. So, every time you reset the Xebec controller - which includes any press of the BREAK key on the host BBC micro - it goes back to its default drive shape settings in the control ROM. That's a 5MB Winchester disc, which isn't really very handy!