Jim,
Your canoe is typical of a AA-grade Old Town from the teens era. It is very nice, and these are somewhat rare, but the features are typica of a AA-grade short-decked OT fromthat timeframel. The countersunk and bunged seat bolts are normal- a very nice feature of course. You'll also notice that your thwarts are shaped differently from those on your other Old Towns. They are thinner, and simpler in a sense, but they are a really pretty style (maybe it's just because they are unusual).
Your half ribs look typical as well. There is one that's clearly out of place laterally in the pic you posted, but otherwise they look like standard OT half ribs.
Your decks are also typical for a short-decked Old Town. In particular the cutout is the standard shape for an OT of this era. Compare this to other OTs in your collection and you csn see how the intricacy of this deck cutout was gradually lost over the years. Yours certainly isn't the last one from Kathy's last list- the one with long decks- or there would be very clear signs of those long decks.
Your sailing setup also looks normal. The simpler form of the mast step would be typical of that era. The only thing that is odd is the lack of a brass rng in the mast seat. I believe it should have had one, but I don't see any screw holes.
Finally, if this were originally a sponson canoe, that would be obvious. The sponsons were attached with two screws (one about 3" above the other) at every other rib. Removing the sponsons would have eft this double row of screw holes that wouldn't be masked by screws holding a single rub rail in place. In addition, the sponsons would le in the position between the gunwales and the level of your rub rails.
All in all, your canoe looks very original. It appears to be a teens-era AA-grade short-decked canoe such as an HW or CR with half ribs, rub rails, and possibly a sailing rig. Bear in mind that the limited number of digits you're sure about means that this is one of 90 canoes. Keep poking around that serial number area. You might try adding a little varnish to the cleaned stem- this can sometimes make the serial number easier to read.
Now, on a completely different note, did you get my last email about the spruce?
Michael