Because they had a punch and hammer.....any other reason is long lost. Just because it is punched 43 doesn't mean it was done in 43. Probably to sell as a copper '43 cent.
I did it! A few years ago.. I was really bored and I took the 2 number counter stamps and made those incused numbers myself. Finally you have found the Cent. I always wondered where it could be
The reasons people damage coins are as varied as the people that handle them. As was said before. The reasoning is long lost.
I found this in Wikipedia: Countermarking can be done for a variety of reasons. If the currency is reformed, existing coins may be rendered void. In this situation, coins already in circulation could be marked with the new value (according to the new currency system). The life span of existing coins could thus be extended, which might under some circumstances be a cheaper alternative to recalling the coins, melting them and striking replacements.
How could they make these marks? There is no other damage except for a little on the reverse where the marks were made.
A couple of number stamps and a hammer. The only damage on the reverse would be directly opposite the stamps.
That was certainly a more common practice in ancient times. Roman and Greek coins were often counter-stamped to revalue them. I'm not aware of any US coinage that was counter stamped for revaluation. http://www.forumancientcoins.com/numiswiki/view.asp?key=countermark