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When the model A was first designed, there were 10 prototypes, designated model "AA-3" or "AA-4". The number designated whether it had a 3 speed of a 4 speed transmission. The Three speed was not adopted. The Production tractors were originally equipped with a 4 speed transmission, operated by TWO levers. One lever had reverse, 1-3, and 2-4 on it, with the other lever being your "high-low" selector. Starting with sn 499,000 all tractors were built with a six speed transmission. For tractors shipped on STEEL WHEELS, the 5th and 6th gears were blocked out. They were "IN" the transmission, but you could not access them. These tractors ALSO had a two lever shifting system---Reverse, 1-3,2-4 and 5-6, plus "high-low". Beginning with sn 648,000 a single lever, six position shifting quadrant was used. I believe that the very few "A" tractors shipped on steel wheels after sn648,000 were still shipped with 5th and 6th locked out. My 2440 still uses a two lever shifter---Park, I, II, and R plus 1-5,2-6,3-7, and 4-8 AND it has the hydraulic (like a TA) high low as well, giving 16 forward speeds, and I wouldn't try and refer to it a a 2 or 4 speed tractor either.
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