Brass Fitting that fits an 1/8" MPT heater fitting on one side a 1/8" ID tube on the other side. I have seen these in AP-1400 Len Gordon Pack to attach the heater to the pressure though there are...
Barb Adapter 3/4" Barb x 1/2" Spigot , The Glue Side glues inside a 1/2" Fitting which would measure 3/4" and a 3/4" ID tube would be glued over the barb.