Various
The ultimate manhole covers site
British Standard 497 - Specification for manhole covers, road gully gratings and frames for drainage purposes. Cast iron and cast steel

Download the standard from the British Standards Institute site (payment) https://shop.bsigroup.com/ProductDetail/?pid=000000000000084547

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.
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