That is not wear and tear but looks to me like a defective piece of steel was used to make it. I think that a crack started at the edge of the hole that the locking button assembly mounts to, which then travelled upwards along the pressed corner. Once it reached the top, the whole bar would be vibrating and fatigue would have helped the next crack to travel downwards.
I'd go back and tell them that you are prepared to commission a Metallurgist to report on the failure, and that you'll be expecting to recover the cost from them.
I'd go back and tell them that you are prepared to commission a Metallurgist to report on the failure, and that you'll be expecting to recover the cost from them.