So, according to this, Mediobanca is the largest shareholder of UniCredit. I guess it could be custodial, but does explain why they are part of the underwriting group that backstopped the deal. If you combine the Central Bank of Libya, Libyan Investment Authority, and Libyan Foreign Bank holdings, then they are actually bigger than Mediobanca.
At the risk of making a mountain out of a mole hill, UniCredit is the largest holder of Mediobanca (8.7% according to Bloomberg).
Remember when CDO’s all bought each other’s BBB and BB tranches, because no one else would?