You have a logical question. Tachometer, speedometer and instrument cluster have gaskets (I call them gaskets, MB Parts Manual refers to them as Shims). Why does the clock not have one?
I took out the clock from my 1971 280 SL, it did not have any gaskets either. MB Parts Manual does not show any gasket for the clock, though it does show gaskets for all other dash instruments.
It is my guess, and purely a guess, that not having a gasket provides yet another source of electrical grounding for the clock. There is already an electrical grounding connector on the back of the plastic cover of the clock.