"My guess is that it's a template to set distance from seat tube to the outer radius of the rear wheel to assure desired clearance."
I use a similar template in my frame building setup which is very like this layout board. The seat tube angle is fixed, and this template allows me to set the length of the chainstays to a predetermined clearance - ie "fag" paper for Alf Engers style TT frames. I have a template for 700C, 650 and 600 wheel sizes.
Mike Mullett
Reading UK