I heard about the plugs previously.
The new ones have a spring loaded steel ball on the tip of the 3 x knobs. They can be fully tightened.
I'm not sure where you get this idea.
My AP1600 was toppled in a wind storm recently. Fortunately, the only damage is that the head of one of the clutch knobs was bent slightly. I ordered a replacement knob, but haven't swapped it in yet, since we're in a pattern of freezing fog every night. I have the replacement clutch knob in front of me right now. The knob is a solid piece of machined metal, with a flat tip.
You are correct, though, that the knobs can be fully tightened safely.
I've never examined the knob from an AP900 or AP1200, but I believe that they are different in that they have a Delrin plug at the tip. They were susceptible to over tightening.