Mint error or PMD? 1797 SOHO Penny

[coinsandmedals]

I stumbled upon an interesting 1797 SOHO penny. The obverse is typical of a lightly circulated coin, but the reverse has a “Ridge” running through the middle focal. The design details appear to be on two different levels so to speak. If looking at the coin on a level surface from top (“Britannia”) to bottom (“1797”) the design details that occur before the line are at a noticeably higher level than those below the line. This difference can also be seen if looking at the coin in the same orientation but from the left to the right side or the right to left side of the coin when on a level surface. When measuring the thickness of the coin with an old school plastic caliper the top edge reads almost 4mm and the bottom reads closer to 3.5mm. It may be some form of PMD, but I have not run across anything like this before. I am hoping one of the more seasoned expert collectors can lend a hand. I have included several (not so great quality) pictures as well as a visual aid below.

[MarkFeld]

My guess is post-mint damage. That is due to the fact that the line goes thought the area in the lower left obverse and the device, apparently without interruption.

[coinsandmedals]

I probably should have noted that line starting in the left field stops at her hand. There is no connection between that line and the line seen in the right rim. Not sure if that helps. Thank you for responding. 

[Conder101]

Looks like a major die crack to me.  The die probably split not too long after this coin was made.

[coinsandmedals]

46 minutes ago, Conder101 said:

Looks like a major die crack to me.  The die probably split not too long after this coin was made.

I had wondered about that. I may just end up sending this coin in with a few other submissions. Is that something that NGC would likely put on a label or consider when grading the coin? 

[coinman1794]

This looks like one of two things: a broken die or a planchet lamination which separated before striking. Both could result in that line and in the coin being uneven in thickness. NGC would probably not mention it at all if it is a die break. It it is a lamination, they would recognize it under the Mint Error service.

[Conder101]

I think a pre-strike lamination of that magnitude would retain some of the rough surfaces from the lamination, especially on the detail areas.