From what I can see, it would appear those are gum valleys, where the application equipment came in contact with the sheet as the gum was applied. These are not watermarks as you would not see them on a used stamp as they are a point where the gum is just thinner on the unused stamp.
In addition, I believe the only watermarks used on U.S. stamps were the USPS single & double-line watermarks. In other words, every watermark would be one of the letters in USPS. Finally, none of the U.S. airmail issues were ever printed on watermarked paper.
As stampman2002 says, these are due to the gum and are not watermarks. They coincide with the gum breakers that can be seen from the front of the block. Gum breakers were light embossing of a sort done so sheets wouldn't curl in storage.
If you put it in watermark fluid, the lines wouldn't show well like a watermark would on that paper.
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Posted 01/23/2018 5:17 pm
