I can shed a bit of light on at least part of the picture from my experience. I believe Annapolis Rigging previously offered a saddle that used the existing spreaders and so far as I know did not have a thru bar. Bebop has a Hall mast that came with the thru bar. It is shaped like a boomerang and I believe machined from cast aluminum. The spreaders fit snuggly over the tips of the bar and are held in place by one clevis pin per side. Next time we pull the rig to travel I will try to remember to take some photos and measurements. Here is a similar item for the J/24 courtesy APS: http://www.apsltd.com/Tree/d108000/e106218.asp