What is the decal on the seat tube? is it a tubeset decal?
The arrangement of the seat stays and specifically the binder set in the tubes is a feature that many bike manufacturers didn't pursue. This is specifically a trait of Cinelli or Cinelliesque (windsor) brands.
do you have pictures of the fork? (if it came with it)
check to see if there is any manufacturers information on the ends, especially around the front edge of the wheel slot.
anywho, it looks like a great frame!
Me thinks you should post it in the classic and vintage forum, as the lugwork will be a good guide as
to who may have manufactured it. There are many experts who will be able to help you out there.