Watermark and Type Check – Once you have determined that this perf 10 coil is flat plate, your work is essentially done, there is no need to check for watermark, other than to confirm that the stamp is authentic, since both of these stamps are type I and must be single-line watermarked (see below).
|Perf 10 horizontally (vertical coil)||single-line||442|
|Perf 10 vertically (horizontal coil)||single-line||444|
Both of the perf 10 flat plate coils, US 442 and US 444, are often faked, since they are in that gray area in which the cost of certification can often be more than the cost of the stamp. Particularly suspect are well-centered stamps and pairs with guidelines. On the other hand, if the guideline is genuine (use the foil test to check if the line is engraved) and the perforations are real, any guideline pair of these flat plate coils is genuine.
The obvious method of forgery would be to trim a fully perforated or large margined straight edge copy of the perf 10 stamp, US425. A seasoned collector can often tell the difference between a genuine coil and a trimmed fake by feel alone, but the size of the stamp is often the key in determining whether the stamp was trimmed or not.
Genuine vertical coils will measure about 21.5mm wide and genuine horizontal coils will measure about 25.0mm tall. The measurement is made from striaghtedge to straightedge. This is not fool proof, some genuine coils will measure smaller, but it is an excellent rule of thumb. If the coil measures more than .3mm smaller than the typical size, the coil is suspect.
The more dangerous fakes are made by perforating the imperforate sheet stock, in this case US409, since the coil can be as wide as needed and will have the proper watermark. With all flat plate coils, the perforations need to be examined very closely.