..sorry, looking again at my posting it is slightly ambiguous............
For clarity, yes the Swan hallmark is defined in several learned works such as Tardy, but unlike almost all other French HMs I know of, one sees it regularly used in circumstances that are outside of that definition.
As mentioned in the example of the cigarette case, I have seen it used as a control mark on items which were already fully and legally hallmarked via a system equivalent to France's and where origin was clear. I have also seen it used on single items stamped once, sometimes twice, and even sometimes stamped no less than three times. I have yet to find an explanation of these multiple marks.
In summary, 'SWAN' is an odd one !