yeah, it's generally agreed that 2002 were built 5/01 to 7/01 and are the same, mechanically as . .
 

the 01.5. my 4/01 build car is labelled as 2001

Okay, but what's the difference between 'H' and 'M' chips?
 

So basically what you're saying is there there is no true 2001.5 designation, it's just that during the model year people started noticing these changes and we're using the 2001.5 designation for any S4 that has those changs? I think those changes started appearing in cars with build dates of 10/00.

yes. that's what i'm saying. audi is known for their mid-model year changes
 

for a4's i know this is true:

- there were changes mid-98 when they added atmosphere's, changed the cup-holders and some other stuff i can't recall
- there were changes mid-99 when they went with projector headlights and put fogs into the bumpers. the door handles, i think, went to the sleek current look and the side-markers when from square to rectangular.

oh, and i don't know the difference between the H & M chips. probably to deal with some of the mechanical changes such as the removal of the f-hose. what's odd is that 2002 has a different chip as well ('T' i think) and i haven't heard anything about any changes that occured for the limited 2002 run.