Knob crud seems to be a universal thing with older gear. All Swan gear comes with knob crud. Some of it is just from crummy fingers but I also think the plastic used promotes mold growth. I take all the knobs and soak then in vinegar then use a tooth brush and antibacterial soap. I've also noticed that high humidity also promotes mold growth.

651-S1, used many in my AF years. Great radio, wish I had one.