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Self explanatory.
Normally Android 5.1.1 allows to tap 7 times on "Build number" field in Settings->Tablet info.
Unfortunately the "Build number" field is absent in my Settings application.
What else can I do? I need to use the developer mode for rooting
Update: here is a screenshot. I have redacted the SN. Locale is IT
Needed to tap on the Serial number field. This is the answer