DOVETAIL SPLINE GUIDE
Designed to be used across the mitred corners of boxes. It accurately cuts the slots for reinforcing splines, with a 1:6 dovetail angle on one side and a 90 degree edge on the other. If you in the US the guides are available from http://www.highlandwoodworking.com/david-barron-tools.aspx  
For the rest of the world please contact Classic Hand Tools https://www.classichandtools.com/acatalog/David-Barron-Tools.html