Testing and feedback from ARCOVE had uncovered the fact that the original driver's hatch design of the M48 Mod A hull's design (T48 pilots #1 and 2) tanks proved to be too small and made for an uncomfortable seating position for the driver when operating the tank with his hatch in the open position. To rectify this problem a new larger driver hatch was devised that was introduced into series production vehicles as early as possible. Also the driver's front vision block was removable and could be replaced with an infrared IR vision block. A redesigned power pack featured an AV-1790-7B gasoline engine and CD-850-4B transmission. The turret was fitted with the M1 cupola that enclosed the M2HB machine gun. Cupolas fitted on those tanks with early production Mod A turrets used an adapter ring. In April 1953, the Army standardized this configuration as the 90mm Gun Tank M48A1 Patton and originally applied this designation to both the early Mod A hull design and the Mod B hulls currently in series production.