Thanks to http://bbs.tiexue.net/post2_13224151_1.html
Thanks to http://www.hisutton.com/China_Stretched_KILO.html
In the South China Sea, a certain base of the Navy's South China Sea Fleet launched a comprehensive battle of submarine warfare, and found and solved the problem of submarine warfare and anti-submarine warfare in a near-real combat environment.
The exercise was carried out in a "back-to-back" manner between the red and the blue. The two sides participated in a multi-type submarine, multiple destroyers and several anti-submarine aircraft.
A video capture posted on Chinese media on 9th September 2018 shows the Chinese Navy’s stretched Pr. 877EKM KILO Class submarine. The vessel has a hull insert approximately 15 meters long. The increased space most likely accommodates an Air Independent Power (AIP) system to increase the submarine’s stealth in littoral environments. Other possibilities include a special mission compartment or cruise missile vertical launch tubes.
China received two Pr. 877EKM Class submarines in 1994-5, followed by two Pr. 636 KILO Class boats in 1997-8 and 8 Pr.636M KILOs in 2004-6. The modified submarine is thought to be one of the first two Pr.887ECM boats and is based in China’s South Sea Fleet.
China already operates AIP submarines with at least thirteen Type 039A Yuan Class boats in service. Chines AIP systems are reported to be of the Stirling type, similar in concept to those found n Swedish and Japanese submarines. It is possible however that the modified KILO has an experimental powerplant, possibly of anther form of AIP.