China's export submarine designation give you some idea on it's displacement. S-20 is a 2,000 tons submarine. S-26 ordered by Thailand is a 2,600 ton ship. So if your customised submarine has displacement of 2,800 tons, it will likely be called S-28.
Picture at post #641 above is a test-bed submarine called type 032. (Many netizens have on their own initiative give it a designation type 41, which is incorrect). All conventional powered submarines (SSK) built in China are designation type 03X, as in type 039, 032 etc.
Type 032 (mistaken as type 041) is a test bed for missiles, torpedo etc. It is not a frontline combat ship.