Larian updated unique items in Patch 3, reducing RuneSlots from 3 or 2 to 1 in many cases. To apparently offset these reductions, a new field was added, RuneSlots_V1, with a value of between 1 and 3. But I don't know what RuneSlots_V1 actually does.
But these changes were made
only in the .txt files, not the .stats files. You won't be able to view these changes in the editor.