This is a 50mm barrel union o-ring to suit a variety of Davey and Spa-Quip equipment.

Davey AquaReviva Chlorinator
Davey / Chloromatic Ecosalt MES Series
Davey Silensor SLL
EcoMatic Compact & EcoMineral Chlorinator
Davey BMSC & CN Series Chlorinator
Davey Power Master and Power Master Eco Pool Pumps
Davey FG28, FG32 and FG36 Sand Filters Multiport Valves
Monarch / Davey Ecopure / Reliance MK3 Sand Filters Multiport Valves (M8200)
Davey SpaPower Spa Controllers with 50mm Unions
Davey EcoSpa Spa Pool Brominator