Nut driver?

I think you're right. Thanks so much! I'll pick up a metric set.

In case this is helpful to someone else: I ended up having help breaking down the Ikea Stocksund sofa which I purchased used. There were 4 large black plastic bolts securing the arms to the base, which the tool pictured is used to tighten/loosen. The fellow who helped was able to easily loosen them with a crescent wrench.