Balboa Heater Gasket - 2"
 
Gasket for use in almost all flow through heaters fitted to hot tubs
Made by Balboa for their range of heaters. This gasket fits between the end of the stainless tube and the PVC tail pipe 
It has a bead either side to hold it in place and is secured via the split nut union that fits over the tailpipe
If your hot tub heater is leaking at the unions, chances are you need to replace these gaskets