Hey Jim
I did what you said and when I look top to bottom and bottom to top I see high lights of white lines with darker areas between them.
When I look at the stamp from the side I did not see the lines, probably because the lines are brightened up by the light, but definitely see them from top to bottom, etc.
If we think this is not ribbed paper and none of my other 231's and 231xx's have this effect..then what are we looking at..?
Looking side to side

Looking top to bottom

Robert