If the stamp has a double-line watermark it is clearly a fake made from the imperforate Scott 343. The perforations should be examined closely. If the perforations appear genuine, there is a very good case that the stamp is genuine since there is no corresponding perf 8.5 stamp from which to manufacture the fake. The vertical coil is relatively inexpensive and is not a prime candidate for forgery, you should be able to pick up a genuine copy without certification. However, you must pay particular attention to guide line pairs which do command a high premium and are often faked.