Watermark Check - There is no need to check the watermark on this stamp, other than to authenticate (see below), this stamp is double-line watermarked and printed by the flat plate method.
|vertical coil - perf 12 horizontally||351|
|horizontal coil - perf 12 vertically||355|
The 5¢ coils, Number 351 and Number 355, may be faked by trimming the sheet stamp, Number 335 on the appropriate sides or by adding perforations to the imperforate stamp, Number 347. Since the unused copies of 335 and 347 are reasonably expensive in their own right, one wouldn't expect to see that many unused fakes with added perfs, but they do exist.
It is quite likely that the used stamp could be a trimmed fake of a Number 335. These will not measure up however. The vertical coils should measure close to 21.5mm from side to side, and the horizontal coils 25.0mm. This is not a hard and fast rule, but coils that measure less than the desired measurements should be carefully scrutinized. There is a high likelihood that anything more than a millimeter off is a trimmed copy of the sheet stamp.
Be sure to watch out for guide-line pairs, fake guide-lines are often drawn in, but the fake line will not pass the "engraved test" (foil test).
The coil must have a double-line watermark. A quick watermark check will eliminate any fakes made by trimming the single-line watermarked Number 378, or any other 5¢ stamp for that matter.