That particular one belongs (belonged) to Bonnie Ruhland. She did the restoration on the boat and trailer. Her husband Erwin did the engine. They are part of the Florida Chapter.
The design of those trailers is to launch off of a beach, where there is no ramp. The boat can be lowered into the water, even if it’s only a few inches deep, and the trailer removed from above.
Thanks westwind. Over the years I have seen a couple of these but never got the explanation. For years duck hunting "rough" my places have no ramps lucky for an open launching spot. Put clamp on transom wheels (inflated large or center bicycle type cradle) to get to water to launch. Cradle works best. Too old to change all my equipment now since it works. 15′ Grumman Sport Boat -110# Aluminum.