[s-cars] ECU Part No. Question

[Dave Forgie]

S-heads:

I was under the impression that the ECU in all our North American S4s and 
S6s was the same and had the same part number: 4A0 907 551 AA.

A fellow on the S2 forum from Lithuania ("kwlw") has suggested that the 
later S6s were equipped with a factory immobilizer, which makes ECU swap
 to S4 not possible without some cheating (and soldering). However, he is
 saying that an S6 can run with an S4 ECU.  I know I have tried Sean D's
 Dahlback-chipped 97 S6 ECU in my 93 S4 but we didn't try my 93 ECU in
 his car.

"kwlw" further suggests that the ECU part numbers are as follows:

S4: AAN Bosch 0261200465 VW Teile Nummer: 4A0 907 551 AA
S4: AAN Bosch: 0261200465 VW Teile Nummer: 4A0 907 551 A
S6: AAN Bosch 0 261 204 379 VW Teile Nummer: 4A0 907 551 F
S6: AAN Bosch 0 261 203 005 / 006 VW Teile Nummer: 4A0 907 551 B (Automatik)
S6: AAN Bosch 0 261 203 601 / 602 VW Teile Nummer: 4A0 907 551 C
S6: AAN Bosch 0 261 203 603 VW Teile Nummer: 4A0 907 551 D (Automatik)

Any ECU experts (Steve E.?) care to comment?

Dave F.