Various
Standard of the International Organization for Standardization (ISO) that defines the quality of the metal, the strength and the chemical composition of cast metal elements, including lids.

The page on the ISO site where you can purchase the standard https://www.iso.org/standard/66643.html

An example of a cover that carries the standard on the "ultimate manhole covers" site. From this lid the picture that appears in the general section was taken.
There are 3 manhole covers in the site that has this standard
Click for all the covers with this standard

Click for the rest of the standards in the site

The ultimate manhole covers site

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