From a distant past memory R & W Clark used Cascover as original deck sheathing, although canvas was a popular method in that era. I had a Caprice yacht built by them which was just painted plywood. At a guess I would think that the deck sheathing had probably been replaced and is not originai. Be aware that epoxy (and resourcinal resins) will not adhere to conventional oil based jointing compounds used in those days - its essential to clean back to bare wood.