When History Tracking is enabled in SFDC on a Custom Object, the Schema refresh in the Object Sync in Marketo will bring over the Custom Object in the object tree. It can be selected and synced, however it will cause SFDC errors when the background Sync is re-enabled because the SystemModStamp is missing. My understanding is that the SystemModStamp does not exist on History Tracking, as this is intended for only logging activities. This is confusing behavior as it does not make sense to allow the ability to sync the child object, if it is not supported for syncing. Perhaps we could consider removing these from the schema to avoid confusion.