I'm guessing that with many imperforates, telling whether they are true imperfs vs. perforated stamps that have been trimmed can be a challenge. This is certainly true for the Young, Ladd & Coffin private die perfume stamps. I've seen plenty of copies of supposed imperfs that have clear traces of perfs along the cut edge. These two are not that easy:
Are there other means of determining the authenticity?... Perhaps a microscope to look for stress lines in the paper? MRI scan?