Ion Knob (Shock Mount/Knob)
The Ion Knob (also referred to as the "Shock Mount") is a large ribbed cylindrical piece on the Proton Pack. It is connected to the Cyclotron via a hole drilled through the Bumper and then passing into the pack shell, where it is tightened on the inside.
The Ion Knobs used in the first film were actually inflatable metal bellows, while the ones used in Ghostbusters II on the Semi-Hero Packs were resin copies.
This part is believed to be a metal bellow, due to photos of damage on the real prop and Ray's bent version in the Sedgewick Hotel lobby. Other arguments exist, however.
The Ion Knob itself has been reproduced in resin and machined aluminum.