This is the Bilstein B4 rear shock absorber, part number 19-127439, listed for the Audi A3 Quattro. It is a replacement damper for the rear axle of the 8P-generation A3, so it is meant to slot in where the factory unit sits rather than change the car's geometry. Searching for "Bilstein B4 Audi A3 Quattro rear shock" or "19-127439 rear damper" will bring up this exact part.
Choose this shock over a generic damper if you want a replacement that matches the factory ride character of your A3 while keeping Bilstein's build standard. Fitment is the detail to verify before ordering. Application details vary between sources, including whether the listing covers the standard suspension only or also sport suspension, and which engines and model years are included. Confirm your car's fitment with the manufacturer or by VIN before purchase. Owners on user forums commonly compare the rear shock's lower mounting hardware against their car's setup, so check that before you start.
