typically not extendible, or extendibily drastically restricted. Especially, no recursively defined external types (like trees, lists, etc.) can be handled properly be them. These data types have to be build-in into these designs. As a result, they do not meet the extendibility requierement.
Having scanned through many implementations of object stores, the author considers his approach (though simple and straight forward) to be very original for abstract (pseudo) machines used as backends of programming languages.
Most likely, it has never been implemented, because of the scare the costy (T,V)->N operation produces in the heart of every implementor who cares for runtime efficience, and finally choose machine cycles over generality. The author hopes to be able to show, that these extra cost is worth the expense.