This is truly a great sword form, and like the Capn, I have always found these exciting with their distinct associations to the pirates, buccaneers and privateers, myself always hoping to find one. My greatest concern however has always been the obvious case for 'less than authentic' examples which of course lurk about with such lucrative forms.
It is difficult to assess authenticity of examples from photos of course, regardless of how well they are taken, and no substitute for actual handling which reveals weight, balance etc. While it is true that forgers have become more adept, even in the Victorian period the skills were even more daunting, and these reproductions (i.e. Ernst Schmitt) actually entered collections and museums where they remained seen as authentic until recent times.
On that note, I agree with the caveat by AHorsa, not to reveal the nuances etc. so as not to openly instruct forgers with details.
One thing I think seems notable here is that the blade has these markings at the forte, and it seems to me (correct me if wrong) that reproductions seldom if ever have viable markings placed accordingly. Having said that, it does seem there have been cases where examples have been reproduced literally 'from the book'. In most such cases, online notifications of such forgeries are often noted.
I am far from well versed in these areas, but have understood that any sort of overall pitting or corrosion is telling as these kinds of natural reactions in aging do not occur in most cases. The popular favor of cleaning these old weapons defeats most visual assessments of age and authenticity and renders only more scientific methods necessary.
Always admittedly an optimist, I am inclined to think that perhaps the auction description assessed this example as reproduction rather to stay on safe side, but as we have seen, many 'sleepers' have been acquired in such cases.
With that, Rob its nice to see you posting, its been too quiet around here! and you even caught the attention of the Capn! who has also been missed. I very much appreciate the observations entered here by the others offering interesting and pragmatic solutions very much part of always learning together
All best regards
Jim