So it seems it is a backup file, since a new 50unattended-upgrades is present.
A diff of them shows
That is to say, the Ubuntu 14 version had
"${distro_id}:${distro_codename}";
as well as
"${distro_id}:${distro_codename}-security";
which is in the old and the new file.
So I deleted the backup/old file for now.