I managed to recreate a version of this effect with Motion that you can download here.
Be sure to quit Final Cut Pro X first, then do the following:
1. Go to your User>Movies>Motion Templates>Effects folder.
Note- if you don't already have Motion installed you may not have this folder. In which case, first create a folder in the Movies folder, called Motion Templates.localized Then create a folder inside of that folder called Effects.localized The folders should look like this (the localized will disappear in the Finder after you enter it):
2. Create a new folder inside the Effects folder called Custom Effects.localized
3. Copy Letterbox_New.zip into the Custom Effects folder.
4. Open the zip file, which should create a Letterbox_New folder within the Custom Effects folder.
Now, launch FCP X. Check your effects browser, you should have a new category called Custom Effects and in it you should have a filter called Letterbox_New with the old Letterbox parameters restored.
*User is your Mac OS X user account name that you're currently logged in with.