My car is EURO S4 B5 from 07/1999.
The ECU is 8D0 907 551D.

Remember the euro S4 B5 has immo in cluster (probably immo2, I 've never really known: but I'm sure the VIN number is not coded in the EEPROM cluster, only the immo ID AUZ7Zxxxxx )

I need a spare one and 8D0 907 551D is hard to find at a decent price.
Is there any other ECU that can match the original 8D0 907 551D ?
What about a a A6 ECU ? What's about immo (I would want to keep this feature ...)

What is the specificities of the 8D0 907 551D that justifies the D letter compared to C or M or G or K ?

Typically, a dumb example:
8D0 907 551G + bin from 8D0 907 551D can be a replacement of 8D0 907 551D ?

1) And If I bypass this step, what is the consequence technically wise ? Immo2 has not VIN coded in the cluster eeprom, only an immoID, so ?
2) I can unsolder the eeprom from ECU and doing it by hand, right ? Or is there checksum stuff in ECU eeprom ?

Note: I already read the cluster eeprom to retrieve the 4-digits PIN.

If put my current S4 file + ECU eeprom dump in the donor ECU (typically a A6 ECU with immo), should it work ?

I don't really catch where the road blocker is exactly.

APB 2.7 hardware is all the same, 1.8/2.8/4.2 ecu's are different. Any ecu from 00-02 (a6 or s4) will work regardless of transmission type it was originally equipped to handle (just make sure it's a 29f800bb flash chip, not the earlier 400). Just flash the file you want. I have some d-box bins I could send your way.

If you really can't find one, PM me. I've been stockpiling them.

Would a 4B0907551F     0261206378-1037-350239         6mt EU  A6 AJK
 box be ok then? found one of these. though wiki doesnt say much about the 4B0 Fbox?