I think the indicator lamp at far right on the control panel indicates a true Hobart-made 21 series.  The model/serial number tag inside the door opening will confirm one way or the other.  If your machine was made by Hobart, check if they still carry the door seal.  First, you should try a diluted bleach solution on the seal while it's still in place, assuming it's a mildew stain issue.  If it's just built up crud like mineral/detergent deposits, white vinegar and an old toothbrush may help to remove that.

We bought a KUDI-21 (the "U" supposedly a designation for an under counter model) new in 1990 and it did a great job for 18 years.  The only thing it ever needed was a new check valve, a little rubber flap for the drain hose outlet that's a cheap and easy DIY fix.  We left that machine behind when we sold the house. 

I think an owner's manual that can be downloaded for your machine shouldn't be hard to find on line.

The 4-way Hydrosweep wash arm lifts straight up and off for filter access -- there's nothing fastening it into place.  Just be mindful of where the tips of the wash arm are positioned when removing it.  They all need to clear the bulges near the bottom of the side walls to lift the arm off.