Hi Jim,
Your canoe certainly looks like an Old Town- many features fit, and the profile is very much Old Town. The flat floor suggests a Charles River model. But Kathy- what do you mean by "Why fit a "river canoe" as you would for one going out on a lake?"... Old catalogs describe Charles River canoes as being suitable for rivers and ponds, and HWs as suitable for large rivers, lakes, ponds, saltwater... By the early 1900s, the recreational canoeing market was so huge, and makers like Old Town were pretty free with their descriptions, apparently interested in convincing the prospective buyer that the canoe they liked would be suitable for whatever use they chose. Buyers could outfit a canoe any way they saw fit.
This canoe does appear to have CR lines (but photos are too small to see much). And there do appear to be holes in every other rib, but it doesn't look like they were for sponsons. There appears to be a single row of holes (again, photos are so small it is difficult to tell), and these are surely the holes for attaching the rub rail. The sailing rig was likely aftermarket- the mast step is a completely different shape from the usual Old Town style (and the way it is attached is all wrong). Are there diamond-head bolts, or are seat and thwart bolts countersunk and bunged? If the latter, this points to an early date, sometime in the teens or earlier.
Bottom line- it appears that you have an early Old Town in AA grade with rub rails, outside stems and half ribs. There does not appear to have been a floor rack or sponsons. These features should help you narrow down the possible choices.
As for the different woods, all of the trim in a AA-grade canoe would be mahogany, so those pieces that aren't mahogany were probably added later. Finally, the seats- these could have been ordered from the factory mounted up against the gunwales, or someone could have removed the spacers later... spacers were standard, but the company would do what you wanted.
Serial number- if there is varnish still there, have you tried stripping it away with a chemical stripper? If the varnish is gone, wet the stem with water- this will sometimes show impressions much better. Recently I was able to determine the maker of a canoe just from the very faint difference in patina where a deck decal was once applied, but not a trace of which remains today.
Hope this helps,
Michael