I have details for Plone 3 (but note that I know this has changed a bit to Plone 4).
In Plone 3.3 history, everything is stored inside the portal_historiesstorage / repo object. There you have the _shadowStorage subobject.
I found that if you delete this persistent object, it is created from scratch if necessary.
Hope this help will be somehow
Luca fabbri
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