That old STX groupset is nice. It looks like it may be STX-RC, which was 8 speed. I have regular STX on my Trek 750 (7 speed) and, with modern shifters, it indexes as well as any other derailleur out there. It's also got a right firm spring in it...it doesn't flop around like some of Shimano's later derailleurs do. That Batavus is a nice bike!