Rocker cover gaskets constantly changed.
first type- RS Liberty/legacy rocker arm engine
next- 94-97 WRX and Legacy GT
next 97-98 WRX and GT-B/RS-K (there are 2 versions of this gasket, even with a VIN you often get the wrong ones.
next 99 onwards non AVCS, WRX and TT Legacy.
then AVCS engines (there are 2 versions of these as well. (with bolt holes and without)

Someone should really photograph the cam covers, the gaskets and the part numbers to make life easier.