third-party recordkeeper

noun | \ -ˈre-kərd-ˌkē-pər \ | third-party rec·ord·keep·er

1. : a party (as an accountant or bank) summoned to produce records pertaining to a tax investigation of another party

   Note: The original party is entitled to receive notice of and to challenge the issuance of a summons to a third-party recordkeeper.