On a re-check, the exposed edge around the lazarette opening for the heavy hatch doors looks like foam and not like wood. The hatch hinge is screwed to the teak planks (not the core). While gluing plugs into the thin teak where the screw holes had failed a while back, I thought it was an odd place to screw to, but it is consistent with not being able to screw to a foam core. I emailed Mark Halvorsen, no response yet, hope to see him at the Sydney BoatShow next week,(also Rex from Sarca Anchors).
I`ve got whatever core I`ve got, but interesting no one knows of this form of deck on IGs, me included. Thanks to everyone who responded,especially JohnP with an actual core photo, and AndyG with his deck experience. BruceK