Watermark Check: You will need to check the watermark on this stamp.
|Double-line||normal||331||Booklet Double-Line||normal||331a||Double-line||"blue paper"||357||Single-line||normal||374||Booklet Single-Line||normal||374a|
The 1¢ blue paper is actually on a dull grayish paper. The stamp must be double-line watermarked. Fortunately, double-line watermarks are hard to miss, if you don’t see one the stamp is suspect. If you think your stamp is the blue paper variety, you should have it certified. The 1¢ and 2¢ blue papers are common enough that the certification cost may seemingly outweigh the resale value of the stamp, particularly if the stamp is faulty or poorly centered. Still, we would argue that all blue paper stamps be certified, if only for reference.
Once you have a reference copy, it is fairly easy to tell the blue papers from the regular stamps.