UNDERSTANDING VW PART NUMBERS

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hi there am trying to work out what the last part of the vw part numbers indicate.i know the first 3 numbers the model,and the next 3 the area of the car,but dont know where to find out the importance of the last section.
In trying to track down the correct servo from my car,someone here kindly checked ecta?!and came up with 8E0612107E, vw dealer came up with 8EO612107J, and have been offered a good servo, part number 8EO612O15J!!
Can anyone tell me the difference please, and if this servo i have been offered will fit?
Alink to a site that i can put in a part number, and it lists the vehicles it will fit would be great.
Most parts companies dont list this servo,it seems to be dealer only.Cheers Adam