sentences:  By the time the third prototype was started, it became obvious building the boats with foreign equipment would be too expensive, so the design was reworked slightly to use only domestically produced equipment.The result of this modification was the IX-"bis" series and went to mass production in 1936.Initially the first prototypes received the official designations N-1, N-2, and N-3 ("Nemetskaya", "German") but in October 1937 they were re-designated S-x ("Srednyaya", "Medium").