a threaded rod goes through the mast and a larger round sleeve on each side is screwed onto the rod holding it on the mast. the spreader bar slips over the sleeve and is held to the sleeve with a cotter pin. attached is a link to the d&r marine page which shows a picture of the bolt and sleeve.
http://www.drmarine.com/products.asp?cat=151
i take the cotter pin out of one of the spreaders and pull off the spreader in the fall so i can put a tarp over the mast. i did have the threaded rod rust out and it broke in two once while sailing. the spreader on the port side came out. luckily, i did not loose the mast and i was able to motor into a marina to get a new threaded rod installed. just make sure the rod is in good shape as the rod in my 74 oday 20 was steel with a hollow center so it gets weak with corrosion more quickly than a solid rod would have. the sleeves are stainless, so they were in good shape.
