Via Peter Tchir of TF Market Advisors,
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.
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?