On Wed, 22 Aug 2012 11:40:11 +0100, Frank Erskine wrote:

Removing this screw reveals a small O ring around the screw and hole
to the water inlet.

But what is it for?

Perhaps to drain the tap/pipe for cold spells.

You could just open the tap. But some Bib taps have a built in double
check valve this could be to drain the bit between the two valves.

--
Cheers
Dave.