Boat's not close, so pics will come as I can. Here's one of the boat on the trailer. You can see the rear cradles. The trailer has a single roller/axle mount. The cradles have a standard rubber trailer roller bedded with 5200. Then you just mount the roller to the trailer with the regular roller axle. When I keep the boat at the lot, I put the beach wheels just forward of the front beam, then I pull the cradles off the trailer, put them on the 2x4 stands, and put them under the rear beam.

I'll try to get a pic of the double-roller thing, not on my trailers.