Bib Tap - small screw

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.

That would make sense although I have a feeling that one did have a non-return valve and the other did not.

It was the failure of the non-return bit that prompted its replacement. A little O ring that seals it closed was coming loose and I was bu**ered if I could find a way of getting it all apart to replace it.

This is what I believe it to be: to drain the non return valve.

I think it is preferable to use a tap without a valve and install a check valve inside the house to minimise the chances of it freezing? At least, that's what I have done. The taps without an integral check valve (ime, ymmv, etc) do not have this drain nut.

It's also to check that both sides of the double check valve are working (like you do;-). Without access between the two valves, you can't test they both still work.

That is the requirement nowadays - the double check valve must be indoors, as they are otherwise silently destroyed by the first frost.

Fitted one for my brother a couple of weeks ago, and used a separate double check valve on the indoor pipework. The brass casting for the tap had a bulge where the access nut would be, but it wasn't machined out.

The ones on double check valves usually have fibre washers IME. Not seen one with an O-ring like the OP's.

How would you test this? Remove the tap and apply water through the "wrong" end with a hose pipe and if it comes out the drain hole, the first valve is broken; if it comes out the "back" of the tap, both are broken?

I've never heard of testing them before. Is it done on installation or is it supposed to be done annually?

I don't seriously believe anyone does, hence the sarcy comment.