I'd like to pre-define page_template choice in the page attributes when creating a new post in a custom post type.
Found lots to do with adding parents defaults, but stuck getting past that.
You need to create the template files within your theme using the correct naming.
Please see:
It's in the filename. single-$posttype.php, archive-$posttype.php
As heady12 pointed out
Apply template to custom post type
With some modifications to conditions this worked.