__Master.dbs is a template database layout generated by <FC_PRPD_ACE> when it needs to upgrade existing databases to a new version (e.g., when upgrading from V10 to V11). It is a template form of the new database format which <FC_PRPD_ACE> uses during the upgrade.
It is automatically removed after the upgrade, although in some situations it may be left behind. If the server starts and stops correctly, it can be removed.