Must be for the main halyard
Since there is only one of these and it is on the starboard side, I conclude this u-bolt is for attaching the main halyard shackle after dousing the sail.The purpose for this, is that when it rains, water runs down the halyard and can pour onto the main sail. With the main halyard shackle attached on this u-bolt, water from the halyard flows nicely to the deck. It is a convenient place to attach the shackle as opposed to a stanchion or whatever.
Since there is only one of these and it is on the starboard side, I conclude this u-bolt is for attaching the main halyard shackle after dousing the sail.The purpose for this, is that when it rains, water runs down the halyard and can pour onto the main sail. With the main halyard shackle attached on this u-bolt, water from the halyard flows nicely to the deck. It is a convenient place to attach the shackle as opposed to a stanchion or whatever.