In the phrase about meeting Ziyād, what does nominative «ʿAbdu» indicate?

Narrator Criticism: Teacher Verification, Conflicting Ascriptions and Claims of Hearing

• ʿAbd al-Raḥmān joined Abū Dāwūd as a visitor/questioner
• ʿAbd al-Raḥmān was another name for Ziyād
• Abū Dāwūd heard this hadith separately from both men
• The phrase converts the report into a mursal narration