After WW I Steiermark lost 30.000 hectare of wine producing land to Yugoslavia. The so called Untersteiermark. What was left is 10% and is nowadays called Süd-Steiermark.
I was struck by the difference between Styria and the adjoining region to the south of what now is Slovenia. Winery after winery line the fairly well marked wine trails of Styria. The roads are narrow and it sometimes could take a while to find the place you were looking for, but with persistence you found it. Just a few miles away in Slovenia we had a devil of a time finding wineries, and then there was a good chance they were not open. The purpose of the trip was to tour the wine regions of Slovenia. Driving from Vienna we passed through Styria. Awfully glad we did.
Do you know a map showing the difference you refer to between pre- and post- WW I Styria?