"England and Wales Census, 1871", database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:VBXM-7QB : 10 December 2017), Frederick R Knollys in entry for Francis Knollys, 1871.
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DNA Connections
It may be possible to confirm family relationships with Frederick by comparing test results with other carriers of his Y-chromosome or his mother's mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree: