IMHO. Anything earlier than 2000. 2003 is when they started with the Chopper and also began stretching the pro-streets. I think of them in this grouping:
1994-1999
2000-2004 LSD (2003 Bulldog RSD, If I remember right)
2005-present RSD
Here's some more detail that I picked up during my trips to the Wichita factory. The following four links show some of their earlier models, which I posted to have some fun with the forum members.
This is a quote from one of my earlier posts, showing the lineup of milestone bikes they have in the show room. "Here's a night shot looking into the window of their showroom. They are starting a lineup of historically significant bikes. On the left is the #1 limited edition K-9. Next to that is their 20,000th bike, the K-9 that inspired the paint job for the P8RIOT Puppy. The middle flag bike is their 10,000th bike, and to the right is the 5,000th. The black bike with orange flames is their first chopper."
There was a Vintage Classic (had the FL style headlight nacelle)
Vintage Light (Had a Heritage Softail stype headlight and shroud)
Vintage Sport ( Inverted front end and billet headlight IIRC)