What is this valve?

[BunnyW]

Forgive a very basic question, but what is this device? It does not appear to be a normal isolating valve but there are two of them in the feeds to the kitchen tap,

 

[Jblion]

Looks like a double check valve, to only allow water flow in one direction. Usually on mixer taps/showers and the like.

 

[BunnyW]

Looks like a double check valve, to only allow water flow in one direction. Usually on mixer taps/showers and the like.

Thanks!