Essentially, it was a key lock system to ensure that a particular sequence (in this case, valves) was followed.  The concept was that a 'key' would be inserted in the first gate valve box which would allow you to open or close it as needed.  That would 'lock' the first key in place and release the second key which you then took to the next valve in the sequence and opened or closed it as needed and so on.  The 'keys' were unique so they would only fit in the next valve that was supposed to be operated.
We use Smiths and Netherlock interlock systems on all our pig trap valves and manifold valves, you can get them to design any sequence in, word of warning, if you dont mate the locking mechanism correctly to the valve spindle and ensure the locking mechanisms are set up with the valve in the open and closed position correctly they often get problematic and people end up breaking them.

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