What's the difference between PAH-320/T4-SS or PAH-420/T4-SS and PAH-320-FM or PAH-420-FM?

Basically, these parts are one and the same.

PAH-320/T4 has changed to PAH-320-FM,

and PAH-420/T4 has changed to PAH-420-FM.

The only difference now between PAH-320/T4-SS and PAH-320-FM is that the latter is steel, while PAH-320/T4-SS is stainless steel.

This is the same for the PAH-420/T4-SS and PAH-420-FM

PAH-320/T4-SS:

PAH-420/T4-SS