This collage better shows the color differentials between the two sides of both of your chips.

The two images on the left side are front and back of your seller find. The two on the right are front and back of your sun exposed chip. (You obviously did not equalize exposure times to both sides. )

I am NOT prepared to say that your seller find is a function of being printed on brown paper, instead of white, but it is interesting that the two sides are very close in coloration, and if they started off as white and have turned brown because of UV exposure, then that exposure has been fairly equal to both sides. If UV is the cause, what would explain the equal amounts of exposure? (And Yes, I have an opinion, but would like to see other comments first.)

Jim