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I did not know this one either, and it seems even more different from delete/erase/extinguish. I had to look this up; wiktionary says that the unloading sense is actually from a different root (MND lössen, cognate with “los”), which may have changed due to association with the “erasure” sense, particularly in the context of erasure from ship inventories and logbooks.
Also, thank you for the context. This kind of detail tends to be extremely difficult to search for.