How can graders tell if an old draped bust dollar was plugged
Couple years ago picked up a AU 1796 small eagle dollar NGC holder just said plugged. I figured out the die strike from scan on E-Bay. It is a B5 reverse die state c. There is a large die lump on letters I&C in America. The coin has beautiful toning the lump is shiny because it is the highest point on the reverse. I thought maybe this is why they called it plugged.
I bought the coin because it has great eye appeal and 1796 small eagle are hard to find in AU grade. Paid 3,900.00 When I got the coin took it to a coin expert here in Hawaii. He told me that NGC would have known that the shiny area was a die lump and that some repaired holes are expertly done.
When I look at this under 6X led power cannot see any evidence that this coin has ever been plugged. I was wondering how graders can tell plugged coins is there some kind of devise that they use?