The owner of the face has no cravat on, and has opened his tumbled shirt-collar to work with the more ease.
Does this imply that when Mr Wegg came in, this owner of the face had already opened his collar, and his collar was already open?
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It implies indeed that the person has, at some earlier point in time, opened his shirt-collar. The shirt-collar is now in an opened state, and that state continues to be into the present.
If the simple present were used, the person would be opening his shirt collar "as we speak", at this moment.