Timer shaft gear difference ???

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Can someone tell me the difference between 1545-26 single and the 1547-18 (18 to 29 JD) Timer shaft gear ? . Do they have different number of teeth or have the teeth different pitch to match the armature worm gears 1648-26 ( 26-29 single )
1648 - 30 (30 - 34 singles ) 1649 - 18 (1918-29 JD ) , Thanks.