This is the Adaptec ACB4070 disc controller. These boards are pretty rare. They are the standard disc controller board found in BBC hard disc systems, along with the ACB4000, although certain other controller boards are known to work.
These are known as "bridge boards" because they bridge between a Winchester disc and a host adapter. Many controller boards for ST506 type drives are intended for PCs and have edge connectors for plugging directly into the PC motherboard. The Adaptec boards adopt a different system making them suitable for the BBC.
On the left-hand side is the 50-way ribbon cable to connect to the host adapter. Just obscured by the cable is an LED which illuminates for 18 seconds when power is first applied to the board. It waits for 9 seconds for each drive and then goes out, after which it follows the pattern of the connected drives, so it can be useful for being able to tell if the controller board is working as it will flash in time with the Winchester disc light.
On the right hand side are two 20-way connectors and a 34-way connector. These connect to the ST506 type Winchester drive. Each board can support up to two discs so one 20-way cable goes to each drive and a single 34-way cable with two edge connectors is used to attach two drives.
Finally, there is the power connector as the power requires its own power source. It uses the standard 4-pin socket still used on hard drives today.