Hi Tim,
Later boats have ballast fully sealed in GRP and set a little further astern than the cabin hatch. The pivot bolt is supported by two steel plates held against thinner ply boards as seen here on my own Just 17:
The best picture I have of an earlier boat like yours, those with sail numbers less than around #150, appears in an item about Jammed Keels:
This seems to suggest that rather than the metal plates seen on later boats the earlier boats relied on wood baffles that would fill either side of the keel, the space seen in your photographs.