Joe switched from square corners to rounded sometime in the early 1980's,
there is a transition period where (I assume) he was developing his process for routing the round corners.
Even Joe's very early canoes had cained seats, usually machine cain, though a couple have been hand cained.
But, being this was a Sommers canoe, the frames could either have been replaced or maybe they ordered them with webbing as a trial?
I have never seen a number stamped in Joe's decks, Sommers used to mark them with a large "X",
if I had to guess, I'd guess that it is a fleet canoe number for/by Sommers.
Dave, if you have pics of the seats with the webbing and the S/N's, that would be interesting.