This is a close-up of the Acorn host adapter, which is identical to that found in the Acorn Winchester Disc and indeed is described in the legend as "Winchester Disc Host Adaptor".

The difference in the system used in the FileStore E20 and the Acorn Winchester Disc is that the Winchester Disc also requires a separate disc controller board to interface between the ST506 type Winchester drive and the host adapter.

With the FileStore, the more modern SCSI hard disc used can plug directly into the host adapter as the controller circuitry is built into the disc drive.