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Rim Mark
An Asymmetrical mark in the rim allows for precise orientation
of the grid, and aids in the identification of each side.
Centre Mark
More recent grids have an asymmetrical centre marking, enabling
the four quadrants of the grid to be distinguished.
Older grids have a 'reverse arrow' centre marking, enabling
each half of the grid to be distinguished.
Note that the central hole is for manufacturing purposes only,
and may not appear in the actual grid.
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