Unable to answer "What diameter is specified on the sign for this fire hydrant?" – Fire Hydrant – https://osm.org/node/12051080377 via StreetComplete 58.2:

da ist kein H<nummer>

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https://westnordost.de/p/227385.jpg
https://westnordost.de/p/227386.jpg
https://westnordost.de/p/227387.jpg
https://westnordost.de/p/227388.jpg