Rapid corrosion/toning spot on slabbed half dime pattern. On coin or on slab plastic?

I noticed an 1870 J-809 pattern half dime from the Newman collection currently being auctioned by Heritage (Apr 2017, lot 5103, see link below). There appears to be an active area of corrosion or toning at the base of the seven in the date. Same coin was auctioned in same holder in Dec 2013 (lot 3942) and in Mar 2014 (lot 10506), and there is no trace of corrosion at the base of the seven at either time. Seems rather strange for this to be happening in three years inside a slab. It may be something in or on the plastic because the location relative to the seven is slightly different in the whole slab view versus the close up.

Cal

2013 link: https://coins.ha.com/itm/patterns/1...-ngc-cac/a/1192-3942.s?hdnJumpToLot=1x=0&y=0#

2014 link: https://coins.ha.com/itm/patterns/1.../a/1198-10506.s?ic4=ListView-Thumbnail-071515

2017 link: https://coins.ha.com/itm/patterns/1...e-lotlinks-12202013&tab=MyTrackedLots-101116#

 

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I lean towards thinking it on the slab because of the location change. This is very possible given differing lighting angles; I've seen it a hundred times. Had that second image not been present, I'd have a very poor opinion of the last owner.

 

I would assume it is on the plastic. I can think of zero ways for a corrosion spot to move ~2.5mm SW on a coin (unless it's alive ;p). You can clearly see the areas, where the spot would be on the coin in the different pictures, and they look fine from different angles.

 

Definitely on the plastic... check the two obverse photos from the auction. In the full slab, it's directly below the base of the 7. In the detail, it's to the right of the 7 a little above the base.

 

In the 2013 and the 2014 URL, I see an oval light spot from the left side of the vertical of the 7,across it, and on the right side, and in the 2017, it is still there, but the right side has enlarged and is more apparent. Although the 'plastic cause' might the correct one, and a visual of it at an angle should show that, since slabs are not airtight, it is possible that it was put into a different environment which caused the apparent 'spot' to blossom.

 

This coin has been very good to Heritage. They've sold it five times in four years. Brought $7,600 today. It must send out bad vibes that cause the owners to dump it quickly.

Cal

 

There's another black spot that shows up in the newer picture at the lower inside edge of Liberty's right sleeve. This is probably not on the coin, but was dust on the slab when the photo was taken. Why black? The slab was tilted slightly toward diffuse light to get the fields to light up. The result is a very low contast picture where the dust spot was a little dark, since it wasn't reflective, but not noticeable. In post-processing, the contrast was adjusted so that the coin looked right, and the speck of dust became the darkest thing in the image.