The Fluidmaster Airgap 6000 Series Side Entry Delayed Fill Valve
is a water-saving fill valve with a 1/2" brass shank, designed to prevent backflow and operate quietly. It is widely available from various online and local plumbing suppliers. 
Key Features and Specifications
Airgap Technology: Features integrated airgap technology to prevent back siphonage, complying with BS1212-4 standards, so no additional check valve is needed.
Delayed Fill: The delayed action mechanism saves up to 1 litre of water per flush by allowing the cistern to empty before refilling.
Quiet Operation: Utilizes a spiral flow vortex generator for a quiet refill process, with sound levels below 20dB.
Compatibility: Designed to fit virtually all cisterns and is suitable for both high and low pressure systems.
Material and Installation: Features a robust plastic body with a 1/2" brass shank for a secure, corrosion-resistant connection. It is designed for easy, no-tools-required installation and servicing.