An Asymmetrical mark in the rim similar to that found in the standard mesh grids.
Only some of the 'HH' series of fine hexagonal mesh grids have this centre marking. It effectively divides the grid into six distinguishable areas.
The older HS and HH grids also have a circular tab on the rim to aid in handling the grid with tweezers.
Note that no grid has all of the above three features together, the diagram is for illustrative purposes.