The spine as I understand it was the spot on the rod where the layers of cloth that make the rod start and stops .Making a spot which may have an extra layer or two of carbon overlapping.
With the better better manufacturing process’s and the carbon cloth often being cut buy computer the overlap is far far less these days.
When you cast a fixed spool you start with the guides pointed straight up yes ? and when you have a fish on the guides are pointing the other way so your rod is bending both ways .
I have a lot of rods and struggle to find the spine on any of the better ones.
I have just run a line though my solaris and it would work for a freespool …just the butt guide is to big really and I would prefer another 2 at the top but it would work.
My main fishing rods have 10 or 11 guides