of 1956 and 2 in 1957. The first Chinese built unit entered service in 1957 and was designated as the Type 04 submarine chaser. During the deployment of Type 6604 in the South China Sea, it was discovered that the original design for arctic and subarctic use was woefully inadequate for tropical and subtropical regions, with temperatures onboard reaching 40+ degrees Celsius. The Type 04 design was modified to address this problem by adding insulation layers and a sprinkler system on the ammo storage boxes to prevent overheat, and canvas shades which can be removed in less than five minutes when needed. In addition nine more fans were added for better ventilation inside. Cuba: MPK-462 (1954), MPK-464 (1954), MPK-465 (1954), MPK-479 (1955), MPK-155 (1955), MPK-159 (1955) in 1962. Indonesia: MPK-424 (1953), MPK-426 (1953), MPK-427 (1953), MPK-429 (1953) in 1958, MPK-292 (1950), MPK-293 (1950), MPK-294 (1950), MPK-300 (1950), MPK-334 (1950), MPK-382 (1952) in 1963. These six ships were reconstructed specially for Indonesian Navy by project "06" — with rearming by RBU-1200 rocket launchers and accommodating to tropical service conditions. Poland: Czujny (ex-BO-411) (1953), Nieugięty (ex-BO-412) (1953), Zawzięty (ex-BO-417) (1953), Zwrotny (ex-BO-418) in 1955, Zwinny (ex-MPK-291) (1950), Zręczny (ex-MPK-296) (1950), Wytrwały (ex-MPK-344) (1951), Groźny (ex-MPK-347) (1951) in 1957. Romania: V1 (ex-BO-157) (1955), V2 (ex-BO-161) (1955) and V3 (ex-BO-466) (1954) in 1956. Chinese service history Two boats of this class, #271 & #274 participated in the Sino-South Vietnamese naval battle in the Paracel Islands on January 19, 1974, with #274 being heavily damaged; however, #274 was able to make it back to the Chinese base at Yongxing Island for emergency repair after the battle, and returned to Hainan Islands the next day. Despite their obsolescence, these boats remained active well into the mid-1990s. Although the ships are no longer capable of venturing into open ocean, these units remain on the People's Liberation Army Navy’s list of its reserve fleet, actively used as weaponry training boats for naval militia in various military maritime districts in China. Additionally, vessels of this class in Chinese service are used to take Chinese children enrolled in military / naval summer camps and junior military / naval academies for short cruises for patriotic education and public relations missions. Due to the age of these units, however, they are increasingly being used in secondary missions in recent years and according to domestic Chinese media sources, even in this limited capacity,