engine codes (please help)

hi im looking at buying an audi 2.0l bottom end but how do i find out what engine code it is im after the 3A?????

Correct me if i am wrong but it should have part number :

053 103 021b stamped on it ? I read that somewhere and took a note of it when i last went on a scraapy visit , but never found one .

Cheers……….