Guadalupe Victoria During the period of time when bullion values dropped worldwide (in the mid 1870s), there was still a demand for certain Bullion Coins that were deemed to be acceptable in commerce at or near face value. The most sought after of these coins were the Portrait 8R's called "Bustman" dollars by the Chinese. But in the same time frame, copies of other trade coins were made as well. There is VERY little documentation about this operation because it was largely done secretly. You find a fact here and there and need to put it all together piecemeal.
Recently (within the past year or so)
Coin World has had a couple articles on the subject of early bullion forgeries. One article was discussing the 1899 Micro O dollar which falls into the category and the second more recent article was in reference to the US attempt to mint 8Rs in San Francisco. The facts are out there just difficult to locate in a single place.
I have been gathering these facts for some time and do intend to publish on the subject when I think I have it all tied up.
Regarding when coins were chopped - both types exist. There are some cast coins where the chops are cast in place. These are the easiest to spot. Then there are the ones the forgers apply after making the coin. These are better but often the chop punches are not well made. But there are some coins that were chopped in circulation and it exposed the fraud. I have a nice copy of a MINED coin (one of the coins where the center is removed and replaced with lead where the addition of a chop disclosed the forgery.
Unless you examine the chops in person it is difficult to tell. Even then it can be difficult if they are done well.