I have a Kildonan Y-stern sitting in my shop at the moment. 18 foot, 46 wide, 18 inches deep. It is very similiar to a Chestnut and the mast thwart is more Chestnut than Kildonan. However, the planking is red cedar, making it a Winnipeg made freighter.
Mine has tapered ribs, which seem to a be a bit of an anomaly, steel tacks, making it a WW2 era boat. It still had the original canvas on it. We found writing on the transom under the plywood cover plate and a large smear of paint and some writing on the planking on one side although I cannot decipher it. You might find the same on yours.
Ribs on this one are 2 3/8" wide, full 3/8 thick. Planking is just under 1/4". Inwales measure 7/8" square. 10 ribs under the stern stem, 8 under the bow stem. Yours appears to have thin cant ribs right at the bow, which Chestnut had on some of their regular canoes and mine doesn't have those.