About 25 years ago I had a really sublime sestertius of Aelius but it unfortunately had an oval pit on the obverse in the field. A very well known and respected dealer stopped by my table and looked at the coin and remarked that he could place one of these marks into the field and increase its value dramatically. I thought he was joking, but he was serious. He decided not to buy the coin as the pit was a little larger than the stamp should be....saved me from an ethical dilemma. I didnt want to sell it to him if he was going to do that.
So... someone had already produced or is/was able to produce the stamp or inlay which is looks authentic? Did you get the impression he had done it before? How disturbing.
Obviously there is no rigorous date to the end of antiquity. According to Gibbon in his Decline and Fall of the Roman Empire (1776), in 476 AD Odoacer removed Romulus Augustus thus ending the Western Empire. Many historians/classicists use this date as a convenient one to separate classical studies from medieval studies.
Anything can be replicated or duplicated unfortunately. Just like any coin can be cleaned, smoothed, or otherwise altered to make it look better than it was before. But we live in a time now when coins can be easily tracked and we can become aware of such alterations. Sad, but we all know it happens. And yes, I am certain he had done it before. We was well known for altering coins. Keep in mind he was one of the worlds top dealers and historians at the time, a name which all would know (at least then, maybe not now). Back then if he bought a coin from you it was known that you either likely missed a great rarity or that the coin would reappear with a new look and appearance (and at a price of 10 - 100 times what you sold it to him for).
History is quite pliable. Classifications are arbitrary and change more frequently than most are aware. For example, most historians now agree there was no such thing as the 'Dark Ages'. Even with numismatics things change sometimes annually.
I've got a few fairly ancient Lincoln pennies. Would they classify as an unbroken chain of provenance dating back to antiquity ? I believe they were found using a high quality metal detector.
In some parts of the Middle East, North Africa and South Asia coins were valued as bullion and personal treasure. I can well imagine some Persian or Indian family whose ancestors had acquired some ancient gold or silver, put it aside for a rainy day, displayed it on festive occasions, gave away some of them as wedding presents and kept them safe, but not buried, in their homes, where they still are.
Oldest provenance I can report from my collection is from 1695. Ripon hoard stycas. I have a few of them. They came into the possession of the Archbishop of York and stayed with his family until last year when they were sold. Not antiquity, but it is a long time ago. This is probably the nicest one I have from that set. Unfortunately was in the ground from around 850-1695