The image above shows the tops of the Reactor-Ready and Reactor-Ready Duo frameworks.
The hole with a black border is where you insert an overhead stirrer support rod. The black ‘liner’ of this hole is two adapters (there is one half on each side) you can remove to insert larger diameter support rods.
If you misplace one of these black pieces, you can purchase a replacement:
RR121041: I-Beam Stirrer Rod Reducing Spacer
Note that this is a pack of 1 (one half, for one side of the metal I-beam).
For Reactor-Ready Pilot, the part number is the same.