This is a rare, discontinued 3.90 ratio R180 rear diff from a 2004 WRX STI. Also fits 05 model years. This is the only R180 differential compatible with 04/05 Sti transmissions. Came out of a running and driving car with 145k, no issues at all. Minimal surface rust on the housing and fluid came out clean with no metal on the drain plug magnet.