Hi,
Whenever you make a change to a content type defintion in the Workbench, a copy of the corresponding database table is taken and stored with the same name except with _BAK on the end. However, the same thing does not appear to occur with corresponding xml files in the ObjectStore folder. This strikes me as odd, because the two have to be kept in sync with each other, so if the idea of the _BAK table is to allow easy reverting, in case of problems, a .bak version of the file, copied at the same time, would be much better than trying to find a matching version of the file in whatever backups you arrange outside of Rhythmyx.
Andrew.