In addition, three custom Progress M variants were launched to deliver ISS modules Pirs in 2001, Poisk in 2009 and Prichal in late 2021.
All Progress spacecraft traveling to Salyut 6 were launched by the Soyuz-U, and dockings were to the rear port of the station.
Veteran enthusiast Robert Christy[3] suggests this may have been an error due to confusion with a TKS spacecraft which later became Kosmos 1686.
[4] Astronautix.com suggests that the spacecraft may have gone out of control shortly after launch, but then been recovered after the Kosmos designation had been applied.
Progress M1-4 spent 25 days in free flight, prior to redocking with the same port.