Well, at least it seems to have functioned like a letter, and it was found in Baghdad. In the Museo Archeologico Centrale dell’ Etruria (Central Archeological Museum of Etruria) in Fiorenze, there’s an interesting pair of artifacts described as a “rectangular tablet with covering.” Somewhat like finding a letter and its envelope together.
So the writing on both is obviously cuneiform, and the placard states they were found in Baghdad and date to ca. 2055 BC. Sadly, nothing at the museum has any information about what the writing on the tablet and its cover actually say. Maybe they’re official correspondence, maybe they’re a sort of invoice for a shipment of goods, who knows? But given that cuneiform was deciphered in 1857, you’d think a quick summary of the writing on these objects would be feasible.
Do any readers out there have a bit of cuneiform experience?
For what it’s worth, cuneiform was the world’s most successful early writing system — it lasted some 35 centuries, only going extinct in the 1st century AD. Since it left behind no descendant writing systems, it had to be deciphered from scratch in the 19th century (after some 200 years of effort).