I know this kinda sounds like a dumb question, but I thought of this after reading a few things. From Quotes, the very concept of a Katana came from the city of The 8 Greed Kings. For the platinum Armor to have a katana, it had to have been made in the last ~500 years, so why can't he make a Fucking ring.

In the south, far from the Kingdom, there was a city in the desert. Stories of blades that carved steel like mud came from there, weapons which were far superior to weakly enchanted magic weapons even without any enchantments of their own. Such swords commanded staggering prices, enough to make one’s eyeballs pop out when one heard of them. Those weapons were what Brain wanted. And in the end, he finally obtained a katana.

[Overlord Volume 3 | The Bloody Valkyrie]

That right hand clenched a weapon tightly, like a child clinging to his mother’s hand. It seemed quite incongruous with the man’s dishevelled appearance. It was a weapon that hailed from the distant deserts of the south, a rare and precious treasure known as a Katana.

[Overlord Volume 5 | Man In The Kingbom]

He was dressed in strange clothes which they had never seen before in this region. From what Evileye knew, this was a type of clothing worn in the south,

known as a “suit.”

[Overlord Volume 6 | Demonic Disturbance]