Hi,
Just to let you all know what happened then...
I eventually found out that the first owner of the boat, not the one who sold the boat to me, had grounded the boat. It was repaired by a local boat yard but this particular spot was merely glued together since it, most likely, was visible part with strong curved forms, which are hard, laborous, to reproduce with hand lamination. The other places had ridiculous thick repair lamination.
Well, fixing this was really laborous, since it is visible part and if that were ground away the fix would not have looked like the original. So I made a hole to the side of the keel matrix, ground the material away underside until I just had thin layer of material left and the laminated new core from underside. Finally, patched the hole and laminated over. Or at least that is the short story since the whole project took quite many hours and contained lots of details.