Go to the PF site
http://pfsearch.org/pfsearch/ and do a search for the stamp. Copy the images to be used in comparison when you go looking for the stamp. You will need to compare the relative positions of the overprint letters. There is no forgery shown on that site.
Per Robson Lowe, there is supposed to be a big difference in overprint color between forgeries and genuine. But don't take the color images from the PF to be correct.
Per Robson Lowe again, there are many minor differences in the overprint between forgeries and genuine. He also lists:
- E of PER is under bottom right point of W in the forgery. E is almost dead center under the W in the genuine.
- E of WEST is further to the left in the forgery vs. the genuine.
- O of TO relative to the T of ATLANTIC is further left vs. the genuine.