[Den*nis O*Brien]

....in my opinion the cardboard itself raises the "Red Flags". The first 5 photos the OP shows seem to define the cardboard as corrugated. This type of material would be a poor choice to mount something on. The entire deal does not pass my smell test. I am no expert but do have 1/2 dozen broadsides in my collection...none were ever mounted on such a material. I would also point out the sloppy careless way the cardboard was cut on the vertical edge. If I am way off base ...congrats on a nice historical item.