Hi Mauro, I'm pretty confident in saying that this would have been an owner's modification. A long time ago I also saw such an arrangement and thought it would be a very useful adaption. The biggest problem with doing so to my mind is the backstay: it would need to be converted into a split backstay with new mounts either side of the aperture or extended to a new fitting underneath the aperture, requiring you to squeeze around it.
My fear for the split backstay option is that it would put loads on the hull in places not originally planned for which could have unforseen consequences for structural integrity down the line. Extending the stay to a new fitting at deck level would seem the better option, but I don't know whether the layup of the hull at that point would be suitable. Perhaps a backing panel of some size bonded to the inside of the hull and then glassed in would spread the load sufficiently. I feel an expert consultation would be needed before going ahead with such a project.
Best wishes,
Paul
