Todd - order a bolt from Performance catamarans or you should be able to get everything you need from the code stamped on the head of the bolt. Get the code information and look it up on the internet.

I highly doubt they are zinc or even zinc plated. If you see a different texture on the inside of the point of fracture - it's probably only because of the different stresses associated with it's failure. It probably was first cracked from repeated stress over a long time and what was left finally gave way - the two different failures will look differently in the cross section.

You may also be able to upgrade to a harder bolt but you are probably better off just replacing them periodically or finding a way to attach the spinnaker block to the hull just in front of the rear beam to locate the stresses directly on the hull instead of going through the beam and attachment strap first.