Sorry dave, I'm calling you out on that one. I have a load of bats from the eighties that have no concaving and clearly have a spine. The two are not linked.
Well kind of but if you have a spine you can't of have to have some cutting.
Look at a tour a rounded shape it has no concaving because of the convexing.
My theory rightly or wrongly if it has a spine it has cutting to have a spine as it will be raised.
Not splitting hairs and Interpritation but a spine is not natural or should I say fin bigger the fin the more concaving is my termanology
Yes concaving tends to be more central in the two corridors either side and the spine is slightly different but to have a spine cutting wood must have occured.
Otherwise it would be convex backed like a tour and a few others.