The only abbreviations commonly set behind the date is the Diktatzeichen (dictation code). It shows which employee has drafted and/or typed the letter.
Official letters often aren't written by the signee him-/herself, but drafted by a clerk and/or typed by a secretary. In that cases, both usually put a shorthand on it, e.g. their initials, where necessary separated by a hyphen.
In this case, the letter may have been issued by the employee UNT, or it stands for a whole department that sends standard letters.