CBP 4457, What do you list?

A few years back when returning to Canada, I had a particular CBSA agent take exception to my golf clubs and how could I prove where I bought them, even though they were four years old and looked it. Never had that problem before. She said I should register them. A few days later, I took my clubs back to the same crossing and said to a different agent that I wanted to register them. He gives me a weird look and says, " we can't register items that don't have serial numbers", which made perfect sense to me. Since then, those same clubs have been across the border dozens of times.