وُجِدَتْ رِسَالَةٌ، وَعَلَيْهَا اسْمٌ مَمْحُوٌّ.
A letter was found, with an erased name upon it.
In context, wa-ʿalayhā is plausibly analyzed as introducing a nominal circumstantial clause. The suffix -hā refers to the letter and links the circumstance to its antecedent. The prepositional expression ʿalayhā serves as an advanced predicate, ismun as the postponed subject, and mamḥuwwun as an adjective.
The clause describes the state of the letter at discovery, not just a second independent event. A sentence with two coordinated acts, such as finding a letter and writing a name, would call for a different analysis of wāw.