Motor Carriage M18. There were three production contracts for the Hellcat: RAD-563 covered the six pilot models. T-6641 was for the first 1,000 vehicles, and T-9167 was for the final 1,507 vehicles. Production changes The M18 experienced various changes throughout its production, both to refine the design from a mechanical standpoint and incorporate features that would enhance its combat effectiveness. M18s with serial numbers 1–1000 and 1007–1096 (registration numbers 40108110–40109109 and 40144883–40144972) experienced problems with their transmission gear ratios. The same action that standardized the M18 directed that vehicles below serial number 685 be returned to the Buick factory for modification, and vehicles with serial numbers 685 through 1096 be modified before overseas shipment. The rest of the M18s built featured an improved transmission. 640 of the vehicles returned to Buick were eventually converted to M39 Armored Utility Vehicles, and ten into T41E1 command and reconnaissance vehicles (see ). Hellcats with serial numbers 1350 and below (registration numbers 40108110–40109109 and 40144883–40145226) had the naturally-aspirated R975-C1 engine, which produced 350 horsepower. The rectangular ventilation grate for the transmission and differential oil coolers located behind the driver's hatch initially had an angled cylinder-like shape, protruding above the line of the upper hull. Beginning in March 1944, M18s with serial numbers 1351 and above (registration numbers 40145227–40146389) had the internally-modified supercharged R975-C4 engine, which produced 400 horsepower; at roughly the same time as the change in engine type, the shape of the ventilation grate was changed to be flush with the upper hull. The model of 76 mm gun fitted to most Hellcats kicked up large amounts of dust when fired. This was enough to impede the vision of the crew, who had to wait until the muzzle blast cleared to fire accurately again. To solve this problem, a muzzle brake that directed the blast to the sides was standardized in February 1944, but a sufficient number were not produced to allow them to be incorporated onto production lines until the summer. In the interim, M18s produced in the spring of 1944 received the M1A1C gun, which was threaded to accept a muzzle brake, but was not so equipped; in October 1944, a maintenance work order was issued by the Ordnance Department to retrofit M1A1C guns with muzzle brakes, but "in the event, few...appear to have been retrofitted with the muzzle brake during the war." Beginning in June 1944, roughly the last 700