I have only seen round spars on a 12. It may be that your boat, being an "Improved Model", could have originally had rectangular spars. 12-1/2 plans are difficult to come by due to the Cape Cod rights issue, especially so for the late and rare "improved" versions, so that might be a tough path to chase. I'll scour my image library and see what I can come up with, but hopefully someone else on the forum will know more.
-Steve