I am starting the restoration on my 39 Otca # 126926, but somthing is
puzzling me, all of the descriptions of the Otca in the Old Town cataloges
make a point of how stable the Otca model is because it has a flat bottom
that runs well into the ends.
Why does my Otca have a clearly rounded bottom
Wooden canoes will change shape over time so yours may be more round now then when it left the factory. I have seen a flat bottomed Charles River model canoe that now looks like a round bottomed HW because it was left with the seats and thwarts out for an extended period. The ribs 'relaxed' considerably during this time and did not return to the same shape when the rails were pulled back together to put the original seats and thwarts back in. The ribs are now much more rounded across the bottom.
I would encourage you to find someone with a similar Otca and put them side by side to see if there are significant differences. The WCHA Assembly might be a good place to this exercise. Good luck,