Normally the rule of thumb for old vacuums is that they are worth whatever nayone is willing to pay for it. I can tell you that its an unusual vacuum and I havent seen many like it. Are you considering selling it?
Im not sure exactly when these were made, but a good estimate would be from the late 30s to the early 50s, they made them a long time, a very good straight suction machine, lots of them in the South.