VW TDI exhaust flapper solenoid. Commonly called a change over valve, or the N239 valve on ALH TDI engines. This valve is installed on the vacuum line in front of the N75 valve. With oval D shaped plug.
While installing the TDI exhaust flapper solenoid inspect your EGR valve at this time. The EGR valve is prone to carbon build-up and leads to many drivability issues on the TDI engines. Use some diesel purge to help rid the rest of your engine of carbon deposits.