A Brass Tumbler

I surveyed the perimeter of the desk first. Clear. Between two stacks of aged paper, I discovered a small brass puzzle box. Three interlocking rings. No obvious keyway. I tested the resistance of each ring with steady thumb pressure. The third one yielded half a millimeter to the left. A faint mechanical click followed. The seam widened slightly, revealing a notched groove beneath. That is progress. One step at a time.