Here is a little more detail about how they worked:
These machines/phones were place throughout the store in the various departments, the stamping phones were used by sales staff when gaining authorization for credit. Same as todays modern day credit line, but back then it was all internal. The salesperson printed the credit authorization on the sales slip, and away the customer went. The sales receipt then was stamped and the machine perforated it, providing permanent record of the credit authorization on the recipt and a duplicate created for the department to keep. The duplicate was then kept, usually in a drawer under the machine. This machine is missing the small drawer that may have come with it originally.
I can tell you now, you will not see another one of these for sale for some time, if ever. I have scoured the internet only to find one for sale, and it was gone before I could contact the seller. This particular phone is missing the little black tray at the bottom that held receipts or copies. the electrical cord is also cut quite close to the phone. However, this would only be a display piece, and getting a metal plate for the front would be easy if it were the preference of the buyer. I can even have one made for a small fee.