[s-cars] ECU question

[jimk at spotgraphicsinc.com]

Is a stock ECU from a 1993 S4 interchangable with a stock one from a
1995 S6?  How about modified?  I thought they were and can't see why
not, but haven't tried it yet.
The reason I ask is that there are 2 different part numbers and the
dealer is quoting different prices with the S6 one being cheaper.  I am
very confused.
93 S4 ECU 8A0 906 266 B
95 S6 ECU 4A0 997-551-X
Jim