2"mpt threaded tailpiece 417-5140 for split nut.
This is a 2 inch buttress thread to 2 inch male pipe threaded tailpiece.
Oring/Gasket available separately:
711-4030 o-ring gasket
2" split compression nut 0121108020 is a heater union split nut with 2 inch buttress thread connections for 2 inch id heaters.
Replaces other 2 inch plumbing heater split nuts like