Center Plug for Fleck By Pass Valves. PN 11972
The Center plug does not need to be replaced unless it is physically broken at the top where the "arm" attaches. Replacing just the Seal ( 11726 ) will solve all leaks.
Note: "newer" versions of the Center plug have a square hole in them. Older versions did not.
We suppose this is a design "improvement" to eliminate any "vacuum" pressure when turning the by pass handle / plug.