If I recall, the spreader was held onto the bracket by a nut and bolt with the bolt upward to thwart any tangling of lines, sails and so on if it protruded downward. The bracket was generally affixed to the mast with pop rivets. Sometimes it was stainless steel or aluminum but over sized. I am not sure which you have. You can remove them by drilling out the head of the pop rivet and then tap the rest thru as it will fall down to the deck inside the mast which can be removed when the mast is dropped.
If stainless, an ordinary pop rivet gun will not work as you will need to borrow a bigger one for that job. Pop rivets come in various sizes but the ones used were generally bigger. I wish you were close so I could help but at least you have some information to go on.