Trust. In the future, please use flat bed scans, not photographs. Also, we usually need a scan of the back to look for some other features. In this case, however, it is obvious from the front of the scan. The small perf hole remnant is in the red circle below (your picture, cropped and enhanced with added circle). This proves that the stamp originally had perforations on that side, and someone in the past tried to trim them off to simulate a straight edge surface:
