Thank you!! That's useful info!
Unfortunately most of it is wrong.
Because there are error coins where only the mint mark is doubled, I'm guessing those were added in a separate process, right?
Correct! Until 1997 all the dies were made at Philadelphia and they did not have any mintmarks. When a mint ordered more dies a group of unhardened dies of the proper date would be selected and the mintmark punched into them. Then they would be hardened and shipped out. Sometimes the guy might pick up the wrong punch and put the wrong mintmark in the die. now he has three options: 1 they can scrap the die. 2 they can hold the die and wait for the other mint to order more dies and include it in that shipment. 3 they can punch the correct mintmark in the die over the wrong one.
Number 2 would be the preferred option, but if it is near the end of the year it may not be likely the other mint will order more dies. Usually the option chosen in that case would be to scrap the die. But maybe the guy is in trouble for having messed up too many dies already, so they just overpunch with the correct mintmark. There are a few cases of an over mintmark being done deliberately. In 1899 the CC mint was closed as an assay office and some dollar rev dies were shipped back to Philadelphia. These were repunched with an O and shipped to New Orleans. In 1938 if was decided that only Denver would be striking the last year of the Buffalo nickel
, but they still had a could of S mintmarked dies on hand. So the over punched them with a D and shipped them to Denver. The same thing happened in 1955. San Francisco was being closed down but they still had so S mintmarked rev dies. So they punched something like 12 different dies with a D over the S and shipped them to Denver.
Same question with over-stamped dates. How would coins that were already minted one year get over-stamped with a different date?
They aren't. The overdate is in the die, it isn't something they do to the coin. In the early years of the mint dies were made in batches and the last thing done to them before hardening was to punch in the date. Well at the end of the year if production had slowed you might have an unhardened fully dated die still on hand.
Well in those early years die steel was expensive and hard to come by, so you didn't want to waste it. If a hardened dated die was still good they would just continue using it into the next year (even though that was against the law). If the die hadn't been hardened, the date for the following year would be punched in and it would be used. As an example, at the end of 1798 the mint had 8 cent dies on hand two fully dated 1798 and six just dated 179_. In 1799 one of the fully dated dies was repunched with a 9 creating a 1799/8 and one of the 179_ dies was finished with a 9. But 1799 was a low mintage year and at the start of 1800 there were still six dies on hand left from 1798, one dated 1798 and five 179_. These were all overdated with 1800, one 1800/1798 and five 1800/179. Those five dies were used to make 11 different die marriage combinations.
After 1907 the dates were in the hubs used to make the dies and were not punched in by hand and there have been I think only six overdates since then and they are actually doubled dies. At the time it took more than one squeezing of the hub into the die to fully bring up the design. Between hubbings the die would be annealed (heat treated to soften it) and then brought back an hubbed again. If it was paired up with a different hub than the first time you can get either distorted hub doubling, or design hub doubling. If the die is hubbed with a hub with one date and then hubbed with one with a different date you have design hub doubling and an overdate as both dates will show. This happened in 1909 on one double eagle die, in 1918 on a Buffalo nickel
die and a Standing quarter
die, in 1942 on two different dime dies, and in 1943 on a nickel die.