AMB Engine differences
 

I need to replace my engine on a 2002 Audi A4 1.8T Quattro w/auto and when i called around the salvage yard, they'd ask me if the engine was "thru vin 40000 or from vin 40001"? Does anyone know what that means? Also are both engines compatible cause one place says they have a 2004 AMB engine available and I'd like to know. If they are different, what modes would i have to do, if needed