This is a brass bulkhead tank adapter.

Complete with brass retaining nut and 2 rubber sealing washers.

The exposed male thread is 3/4" BSP. BSP threads are not the same as ruler measurements, they vary by about 1/4". Please see attached chart for approximate measurement conversions.

Connected to the adapter is a 1/2" plated brass lever tap with 1/2" brass hose tail (suitable for  1/2" (ID)  garden hose pipe.

FITTING

Firstly, you will need to drill a hole (25 / 27 mm diameter) where you want to locate the item.  

IMPORTANT.....You must be able to get your hand into the tank or drum somewhere close to enable tightening of the back nut.  You will also need to make sure that the area where the hole is being drilled is reasonably flat.  

You will not get a good watertight seal on a heavily curved surface (a slight contour will be OK).

Once you have drilled the hole, remove the back nut and rubber sealing washer from the adapter and push the threaded section through the hole.  

Then push the rubber sealing washer back over the threaded section and finish by screwing home the back nut as tight as you can by hand.  Do not overtighten.