On a curved surface, no less! I have another socket on which someone tried to stamp their initials, but the steel was too hard and the stamps barely left a mark. That's what makes me wonder if it was stamped at the factory.
I tried a little searching using "R U N" in the description, but didn't get anywhere. I wondered if it might be military markings, but hit a dead end there too.
I like the broad well defined knurling and appreciate half inch sockets that go down to smaller sizes. If R U N is someone's initials, U is an Uncommon first letter for a middle name.