Soyuz 7K-OKS
The last two Soyuz space craft of this series were of the designation Soyuz 7K-OKS. In contrast to Soyuz 7K-OK, which docking mechanism lacked an hatch for internal crew transfer, the Soyuz 7K-OKS spacecraft were modified to utilize the new Soyuz "probe and drogue" docking mechanism that allowed internal crew transfer – this was done successfully with the manning of the Salyut 1 space station by Soyuz 11. This probe and drogue docking adapter is in use until today at the ISS.
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