This is an Adaptec ACB4000 Winchester disc controller board. Early hard discs had separate controller boards like this whereas nowadays the disc controller circuitry is built in.

The 50-way connection on the left goes to the host adapter board underneath. The disc itself would be connected up to the sockets on the right-hand side of the board. There is a 34-way connector just like on a 5¼" floppy disc drive but there is also a 20-way connector. There is a second 20-way connector as this Adaptec board can support two Winchester discs.

The type of discs required for the Adaptec board are the old ST506 type which have the 34-way and 20-way connections. They can be difficult to get hold of nowadays which is a bit of a shame since the disc is missing from this unit.