Odd "Grill" On US 190 Back

Purchased a US 190 from Kelleher that came with a PSE cert. Examined the back to find the faults mentioned on the cert. Noticed a regular pattern of indentions or projections that do not look like any of the US grills.
US grills I am familiar with have points in vertical and horizontal rows.
This item has "points" that are much longer horizontally than they are tall. And the
alternating vertical rows are offset.
What would have caused this pattern on the back?
(the black dot at the bottom of the photo is a spot on my lens)

If the stamp is used, it could be an imprint of the (album)page it was on?


Peter

Sandman,
To get the most meaningful responses, I would recommend a full scan of the front and back, then any close-ups to emphasize the specific question. And in this case, also an image of the cert. You might be surprised what the eagle-eyes of the readers here can find.

Hi, I believe this to be a normal pattern/weave of ABN paper, when compared against other examples.

Agree with Sergio
Pattern seems to be on most of stamp including upper left
Normal pattern of ABN soft paper
Better scans would help

part of Becker's request is attached, the cert

I appreciate the suggestion that the pattern may identify the ABN paper.........