Thanks to don b, Scott and Jason Grooms for suggestions. Here goes:
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Serial number on tranny:
9N 2158 has "H 1939" on the tranny. Looks to me like a build date code and not a serial.
9N 4394 has "4202" on the tranny
1940 9N's 13836 and 21776 have nothing that I can see on the tranny
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Squared off engine cradle (also called front axle support assembly).
Jason Grooms told me that this cradle is noticeably squarer on early tractors (he owns 9N287). I couldn't see a difference with the above 4 tractors. The unit is TOTALLY different from the one on my 2N, though, which has a different part number entirely.
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Early trumpets have shorter studs and the housing looks different at the hub, per Jason Grooms.
This may apply to the very earliest tractors. I could see no difference in my group of 4.
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Double rib fenders - Jason has noticed slight differences in the "fit" of the support bracket. Basically, the fit got worse as time went on.
Both of my '39's looked identical and were originally fitted with 16 rivets. Jason has also noticed differences in that some fenders appear to have had rivets left out.
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Scott mentioned to me that the shape of the cast-in support boss where the lower lift arms attach changed from rounded to square on the front.
I checked and both of the '39's (2158 and 4394) have the rounded bosses while the 2 40's have the standard square front ones.
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The left front of the oil pan has a notch where it fits the block on early tractors.
In later tractors the metal edge runs straight. Note that the block is still notched on later tractors, but to simplify manufacture the oil pan is not notched as notching it serves no purpose but to save maybe a pound of cast iron.
Both of my '39's and both 40's have the notched oil pan.
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The early 9N's had the post ammeter rather then the loop ammeter, and the bridge in back ran up and down rather then horizontally.
The ammeter on 9N2158 should have been the post type but it has a replacement ammeter, unfortunately.
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