Lids on the wall instead of on the ground are quite common in Portugal, for example
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In the photo from which the lid was taken, you can see the wall and the lid on which Click for a larger image
In the next photo in the gallery, taken in the same city, we see a collection of lids on the wall Click for item in Gallery
GJS-500 also called GGG50 marks a strong and heat-resistant type of metal while GJS is a sign for Continuously Cast Spheroidal Graphite Iron - which describes a process in which the metal is continuously fed.