Control Unit for Magnetic Tape Drives.

Front view with door open.

The control unit relieved the CPU of the nitty gritty detail
of controlling the operation of the tape drives.

The cables you see are:
The fat one connects to the channel upstream,
and the thin one to the tape drive downstream.

A rear view with the door open.

The thick cable coiled at the bottom is the power cable.

There are five backplanes that have circuit cards on the other side.

The panel on the top left allowed Customer Engineers
to run diagnostics on the tape drives.

(Customer Engineers what were they?
They were employees of IBM who had an office/workshop on-site
where they kept their equipment/spares.

The mainframe computers needed daily/weekly maintenance
as well as upgrades,
and of course needing repairs when they went wrong.)

This is the engineer's control panel, if it is viewed Fullsize you can scroll around and near the centre find:

-	ROS Address
HI ROS	ROS Data Bits 0-7
Lo ROS	ROS Data Bits 8-15

ROS is an acronym for Read Only Storage, this means firmware was in the Control Unit.
Firmware is sequence of instructions that are decoded and executed by hardware to perform some function.
In fact an engineer could run diagnostics on a tape drive whilst other tape drives were in use.