Take another photo straight down. How many edge slots? 1 or 2? Most plugs are divided into 4 quadrants or in thirds. two pieces are less common, one piece is very unusual.
Hmmm. It might be this one, found via google image search:
That's very odd, with a spring to tension it. Strange. It appears to only have one slot.
Like the previous comments, a drop of oil in the center part to lube the interface between the outside collar and the center "wedge". Then insert the center bolt and lightly tap it down, the center "wedge" part should pop loose.
Then throw it out and get a normal one.
Perhaps missing the top washer and spring: