A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... - Ayrton Senna, (Brazil) 1994 - Michael Schumacher, (Germany) 1995 - Michael Schumacher, (Germany) 1996 - Olivier Panis[?], (France) 1997 - Michael Schumacher, ...