Watermark Check - You will need to check the watermark on this stamp, however all the perf 12 stamps were printed by the flat plate method and there is no need to check printing method unless the stamp is suspect.
|double-line (blue paper)||360|
The 4¢ blue paper was never issued for general use. Only one pane escaped destruction and, according to Johl, most of this pane of 100 are known, all unused. If you think your stamp is the blue paper variety, you must have the stamp certified. The 4¢ blue paper is one of the great rarities of 20th century U.S. postage stamps.