The 2.1b version adds an upgraded engine (RD-0124) with improved performance to the second stage. First launch took place from Plesetsk Cosmodrome Site 43 on 26 July 2008 with a classified military payload. The 2.1b/ST version is sometimes called Soyuz ST-B.
Progress Rocket Space Centre, formerly known as TsSKB-Progress, is a space science and aerospace research company which is known for manufacturing launch vehicles and satellites. Most notably, Progress Rocket Space Centre is the manufacturer of Soyuz launch vehicles.INFO WIKI