Hard to tell from photos, but the only reason I can think of for the four spots on the back to be sanded down would be if there were once threaded studs for mounting. The smoothness of the letters is not uncommon, as I have several signs that I know to be authentic that do not have "heavy shelving". There is not enough here for me to form an oppinion, but I do not see any red flags. I would say it is definately worth a closer look.
A picture may be worth a thousand words, but holding it in your hands and seeing it in person is worth more than a thousand pictures! Hope someone else can be of more help.