The discovery was made around Manwoltae in Kaesong, a site of royal palace of Koryo.

Three of the metal types are of cuboid shape, 12-13mm wide, 10-11mm long and 6-7mm high. Each has an embossed letter on one side and a bead- or dome-shaped groove on the other.

The last type is 7mm wide and 6 mm long.

The metal types were made of bronze.

After all, a total of seven metal types dating back to Koryo Kingdom have been discovered up to now in the country.

The first was said to be found on a Koryo royal tomb in Kaesong before the liberation of Korea from Japan's colonial rule.

The second was unearthed in a site 300 meters west of Hoegyong Pavilion in Manwoltae in Juche 45 (1956). It is now preserved at the Korean Central History Museum.

The third was discovered in southern part of the western architectural group in Manwoltae in November last year.

The recently-discovered four metal types are similar to the previously- found ones in shape, size and quality.

It was ascertained at a joint symposium of historians, archaeologists and national classicists of the DPRK on April 20 that the newly found metal types date back to the 12th to 13th centuries.

This fact goes to clearly prove that Koryo Kingdom was the first in the world to invent metal types.

Ro Chol Su, vice-director of the National Authority for the Protection of Cultural Heritage, said that more historical relics have been unearthed in the Manwoltae area to enrich the Korea nation's house of heritages.