ProRadiant™ 56169 PureFlow® Hydronic Mixing Block O-Ring, For Use With Model 2887.3 Hydronic Mixing Block, 2887.33 #3 Hydronic Mixing Block Delivery Fitting, 2887.32 #2 Hydronic Mixing Block Return Fitting, 2887.31 #1 Hydronic Mixing Block Supply Fitting, Import