Renault records make no distinction between 1917 and 1918 output; the decision to arm FTs with a 37mm gun was made in April 1917, before any tanks had been manufactured; because of various production difficulties and design requirements, a range of turret types were produced by several manufacturers, but they were all fitted to the basic FT body without any distinguishing reference; all FTs had the same model 18 hp engine. The Renault manual of April 1918 is entitled "RENAULT CHAR D'ASSAUT 18 HP", and the illustrations are of the machine-gun version. The official designation was not changed until the 1930s, when the FT was fitted with a 1931 Reibel machine gun and renamed the "FT modifié 31". By this time, the French Army was equipped with several other Renault models and it had become necessary to distinguish between the various types.