One of the mysteries of the English language finally explained.
At Oxford or (rarely) Cambridge University: (the title of) the person appointed to manage and maintain the archives or records; the archivist. historical after 18th cent.
Early 17th century (in an earlier sense). From post-classical Latin custos archivorum keeper of records to the Lord Chancellor, keeper of the archives at Oxford University from classical Latin custōs + archīvōrum, genitive plural of archīvum.
custos archivorum/ˌkʌstɒs ˌɑːkɪˈvɔːrəm/
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