I am several hours in to trying to implement and still getting stuck.
Used the built in migration.
I have turned off the default instance.
Installed the given code in my function.php.
Added a custom post type to the post-type array. [I assume this is how we now need to implement for CPT's as the previous settings list is no where to be found.]
Put code in my theme file, code that works just fine on a page in a different site.
Now I have attachments on the edit screen, with my user defined meta box, but not on the front-end post view.
And, on a different custom post, old attachments visible on the front-end post view but not listed in the meta box on the edit screen.
I would appreciate a hand or a tip