Before I hazard a guess, I wonder ...
Does your N have an oil line to the governor? What shape are your radius rods, I-beam or tubular? Where is your transmission dipstick, on the gear shift cover or on the right side inspection plate?
I could be wrong, and please don't take offense, but I think it's possible your SN is 221040. To my knowledge, Ford did not stamp any letters on the N serial numbers (other than N of course.) Their font is wierd, though, and the 1s look like Is. Also, the SNs are hand-stamped so the spacing is not uniform.
I'm curious; let us know the characteristics above if you don't mind. These small design changes can confirm an approximate age -- if they confirm my fear that your SN is 221040 then you can be fairly sure that's it. On the other hand if they confirm an early 1939 model then you indeed have a very early model and we have a puzzle with what the I and 040 mean.
Look forward to your reply,
Pete P.
Harborcreek, Penna.