Kubota calls it the "split pin plug" From the manual:
The tractor is equipped with split pin plug under the clutch housing. After operating in rain, snow or tractor has been washed , water may get into the clutch housing. Check by pushing in the split pin. If water enters into the clutch housing remove the plug and drain the water, then install the plug again.
When I had an L3400, I checked it many times, and never saw a drop of water come out of it.