The VIN numbers of the 1988 cars are almost as confused as the early 1983 ones. according to Audi, the Series 2 MB engine was fitted from 85-J-900 644 - but at least fifty later cars still have the earlier engine, not including those that have had engine swaps.

Audi ur-quattro 1988 VIN Breaks.

from vin	change
85-J-900 001	Series 1 MB: MB engine derived from USA MC-1 engine. ECU controls fuel and boost. Knock sensor provides feedback to ECU. ECU stores diagnostic codes and can exercise components. Hydraulic (self-adjusting) valve lifters Injectors are air-shrouded for better pattern. Active idle stabilisation via separate processor. Water jacket on turbo bearing with electric afterrun pump. Tandem hydraulic pump. Orange digital dashboard. Torque-sensing (non-lockable) centre differential.
85-J-900 322	Rear tie rod changed.
85-J-900 337	New clutch plate, flywheel and dowell pins.
85-J-900 644	Series 2 MB: Tracking change in USA to MC-2 engine (except 2nd knock sensor). 142-tooth supertorque cambelt. Skirt and fuel drain on inlet manifold. Boost for wastegate control throttled. New oil pump.