Unknown error, please help a dummy =)

Hello everyone. I hope you are all well. So I've been sorting though a bunch of coins and I came across this. 

It's a 1998 Italy commemorative 500 lire. On the face side, there is a noticeable drop in height by about half a millimeter. You can physically feel the drop with your finger and in the right light, it casts a shadow. Yet the writing and star on the edge seem to be as they should (notice line runs directly through star) 

I have a few questions I'm looking for answers for, Facebook hasn't been much help in the coin groups so know I need to seek proper experts. 

Has anyone seen this thing before? What's the error called? Does anyone know how it happened? Do different mints use different processes (this is Italian) and where would be best to get an idea on value? 

I plan to get this graded. But want to be sure its worth doing as never done before and it's quite expensive to do a single coin 

TIA for any information you can give me 

Matt 

Looking like a superimposed coin of the same diameter as the original, could be PMD (Post Mint Damage).

With close inspection pal. You can see the star on the edge has been stamped after. The coin is in almost perfect condition with no marks on other side whatsoever. The finer details on the lowered part are still raised. Definitely not done post mint 

I also thought another coin could have done it. But the line isn't a perfect circle. Under the star, it changes direction 

I've seen errors like this before. I have an error coin like this in my collection on a 1920's Chinese 200 cash coin. I don't know how this happens or what this type of error is called, but I'd be interested to find out too.

Really, could you upload a picture of it please. I have been searching high and low last few days on net and cannot find any other examples or even a name for it. 

I have sent a few pics of mine to an auctioneer to see what they might know about it. I'm assuming if this has been something before then they would have seen it. 

Is yours a perfect circle. Mine isn't, it changes where the star is placed on the edge. Which makes it even more wierd