The postal code 1090 is the Bulgarian postal code for the post office at St. Kliment Ohridski Base. But there are no base markings on this cover. It could have been posted to me from Bulgaria and not returned from the Antarctic continent. I sent this request late last fall. It was postmark in January and just got to me yesterday. The turn around time is certainly long enough for this to have come from the base.
St. Kliment Ohridski Base has had a post office since the mid 90's. The base is located on Livingston Island in the South Shetlands, at the tip of the Antarctic Peninsula. It is not far from the Spanish base "Juan Carlos I". The base accommodates about 25 people during the summer months and is a routine stop for tourist ships sailing through the area.