I know exactly what you are referencing about the extension which can be flipped either way.
I have also read (somewhere!) that quilt slipcovers about the finished twelve inch size were made like a long pillowcase and basted to the upper portion of the quilt. It reminded me of dollies made to protect furniture arm rests or backs of chairs at the headrest area. These removable covers were easier to clean than the heavy wool quilts made from suiting samples.
Thanks for sharing :D