[Keysh]

It sounds like they were excavated in 1881-1882 (e.g., https://en.wikipedia.org/wiki/Sippar#Archaeology) by Hormuzd Rassam ("widely believed to be the first-known Middle Eastern and Assyrian archaeologist from the Ottoman empire."; https://en.wikipedia.org/wiki/Hormuzd_Rassam) working in conjunction with the British Museum.

"From 1877 to 1882, while undertaking four expeditions on behalf of the British Museum, Rassam made some important discoveries. Numerous finds of significance were transported to the museum, thanks to an agreement made with the Ottoman Sultan by Rassam's old colleague Austen Henry Layard, now Ambassador at Constantinople, allowing Rassam to return and continue their earlier excavations and to 'pack and dispatch to England any antiquities [he] found ... provided, however, there were no duplicates.' A representative of the Sultan was instructed to be present at the dig to examine the objects as they were uncovered."

So, not a euphemism for "stolen".