Hi,
This mystified me for a while but here is the low down on Mk3 arch trims or no arch trims;
All GTIs have arch trims
VR6 from launch to end of 1995 model year had arch trims
From 1996 model year onwards (approx Oct 05 registrations onwards) arch trims were deleted but the sill extensions remained - you can tell a 1996 model year N reg by the oval side repeaters rather than orange.
Curiously the Highline came out just before the standard VR6 lost its arch trims so there are Highlines on an N reg with orange side repeaters and no side mouldings. Also no genuine Highline should have the sill extensions.
No other models had arch trims apart from the limited edition Golf SE (1.4 or 1.

1996/1997 which did a better job of looking like a GTI than the bare arched driver.
Hope that's put your mind and rest without sounding too geeky!
Andrew Chapple
Volkswizard