Thanks Harold. The piping has a nut, with a lip/flare on the end of the pipe (so the nut won't slip off). These nuts screw onto the exterior threads of the PRV. I won't be using the interior threads of the PRV, only the exterior threads.
I'm just overly confused with the terminology. What is a union connection? Is that the existing nut that is on the pipe?
I just know that I need EXTERIOR threads on the PRV. I'm just not sure which Watts model I need (N45B Water Pressure Reducing Valves , Water Pressure Reducing Valves - Standard Capacity, Water Safety & Flow Control - Watts).
Thanks,
Gary