What does "keyboard" mean in this context?
You know it, sir. Defiance, maybe, but, well, no more. The point is, a marine is on guard at the keyboard, just round the corner of that passage. Night and day. Bound to have seen him. He denies-----"

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Keyboard is probably quite literal. We often use "keyboard" to refer to a computer, screen and keyboard. From the context I cannot be 100% sure, but I think it should be the case.

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Maybe that marine is a hacker, or he is on guard through monitoring! I am not sure!