The ship left Australia on Dec 3 and reached the Showa research station on Jan 6 to support the 61st Antarctic research expedition during the 2019/20 season. Apparently it returned to Japan sometime in March.
I just discovered that the year in the the date of Japanese postmarks is the number of years into the current era. With the ascension of the new emperor last year, Japan is now in the Reiwa era. The date format is year, month, day so the first "2" in the date stamp means 2020 on the Georgian calendar.