By The author in the site explains KK for Karna also nicely.
He says since she was sending of the baby alone she must have used a gold armor and earrings. Armor to protect the baby and with earrings and other jewellery so that whoever adopts gets reasonable amount of wealth to raise the child. It may have some special emblem on it.
Adiratha is from anga. Which is known for its iron ore deposits even now. Presumably he must have got the metal from here and used it build a new armor with the same emblem as it was in the gold armor.
He mentions that use of iron had just started and very few people had access to it. Other Warriors were probably wearing wooden ones covered with brass or copper, why so coz if pure brass or copper is used it becomes too heavy and wooden ones with bras or copper plates in front cannot be as protective as iron armor which is lighter and stronger compared to the two metals, it is supposed that the sudarshana chakra of Govind was also made of iron compared to other weapons which used alloys of copper. Kunti recognizes the emblem and hence Karna. Yes he gets it from his adopted father. And it makes him quite invincible in battle unlike other warriors hence Brahmin asks it as a donation.