Pair of 2.0" flow-thru heater split nut unions. They are the most popular split nut unions. The overall diameter of the split nut is 3-3/8" and the flange side opening is 2-1/4". They are made to replace heater flange nuts. They have a large flange for stainless steel housings, and these split nuts are for heaters only. These split nuts will fit; Brett Aqualine, Balboa, Cal Spas, Gecko, Gulf Coast, Hydro-Quip, and many others with metal 2" heater assemblies.
Note: The 2.0" is not the actual measurement but a representation of the inner diameter of the plumbing that is used.