Suspect Scott #352.
If real this is a modestly expensive stamp with a Scott Catalogue value of $190.00 used. The Experts Book states that the #352 is often faked by adding perforations at left & right and trimming top & bottom a Scott #343, adding perforations at left & right to a Scott #343S or trimming perforations off at the top & bottom of a Scott #331.
A check of the watermark reveals a reversed double line mark that reads horizontal. The horizontal watermark eliminates the possibility of the suspect stamp being a #331a booklet pane single. The image below is a photo of the watermark taken with my Safe Signoscope watermark detector. The double line "S" can be seen in the lower left corner.
The height of the suspect is 24mm, 1mm less than the ideal height of 25mm. According to The Experts Book height is not a determining value for the #352.
Now for the perforations. I did a flip comparison of the perforations and they line up very well - see image below.
I also took a look at the cancels and how they look when they cross a perforation and in all cases the cancel ink goes all the way to the edge of the perforations - see the images below. All this together tells me that the perforations are most likely genuine.
The cut edges are next, I've looked both edges over very carefully and can find no evidence of them being trimmed down, there are no indications of perf pits of any kind.
In conclusion: I'm going to go out on a limb and say that the suspect stamp is in fact a genuine Scott #352. I'll be sending this stamp out for expertizing with the A.P.S. in the next day or two.
Reference material used:
As always comments and suggestions are always welcome.
To Contact Riverside Stamps: