see also:   Private Perforations - Vending and Affixing Machine Perforations

Caution:

The 4¢ imperforate, Scott 346, may be faked by trimming the perforations from any of the 4¢ sheet stamps, although to be double-line watermarked it would need to be a Scott 334. It is unlikely that the mint stamp would be faked since the mint perf 12 double-line watermarked stamp actually has a higher catalog value than the mint imperforate stamp. This is not the case for the used stamps however, and care must be taken to insure authenticity. Collecting in pairs is always best, although margin copies or four large margins usually insure authenticity. Remember the 4¢ imperforate stamp must be double-line watermarked to be genuine.