The field should only accept values from 39 until 47.
It can be empty.
It should be editable on the admin member edit page.
It should be visible on the public user profiles.
If you've opened any of the previous hints you might have noticed that we've been looking at the analysis we did before. So we're doing this once again with the 6th bullet: It should be editable on the admin member edit page.
This bullet says that the field we just created should be shown in the administration panel when editing the profile. Usually this is really easy in Django (it's automatically done once you add an admin for a model), but the profile is a bit different since it uses an inline with the fields specified.
You should be able to find it and add your field using this information. You can find a spoiler here.