Success! After installing V6.25, and copying the EXB files from my backup over the local EXBs - "voila!" my old data is back!
So clearly this local data was not syncing with the Evernote Server. Two additional questions to get me running on the latest supported version:
Why didn't the V10 install detect the local notebooks and convert them to sync'd ones? Do I need to do something during the install?
How do I pull the V6 data forward to V10, assuming the install doesn't detect the local notebooks?