I just came across this from a link on another site. I find the idea intriguing.
A short description from the catalog.
The TT-1 Turning Tree works in conjunction with your outside calipers. In the past, you would set your calipers to the diameter you wish to turn by laying them on a ruler and adjust accordingly. This method is full of problems and can cause undersized turnings. Obviously, oversized turnings are easily fixed, but they take additional time. The other problem exists with the calipers themselves. Individual sections can be removed and kept in your apron pocket as quick references until your turning job is completed. All sections nest together, sturdy as a tree, awaiting your calipers.
The site says it is no longer available for sale, but don't you think we could make one for ourselves?