Clear Plastic Ring On Bottom Of HydroFloss Is Missing?

The only two replaceable pieces on the bottom of the HydroFloss reservoir are the white plastic insert and the black o-ring. 

If you are seeing that a clear plastic ring is missing, it's actually broken off and not missing.  It is one solid piece with the rest of the reservoir.   (you can see a picture here.)

There is only one hydrofloss model at the time of this writing.  You can find the model number stamped on the bottom of the HydroFloss.  It is:   7618.034

 

Replacement Reservoir