It is a copy which they may have needed because they had lost the original or, as today, they may have had to send it off for some purpose. As to the stamp, stamps were often stuck on things such as bills. I remember our electricity bill always having a brown 2d stamp on it and the "paid" ink stamp put over it. I think it probably goes back to the days when certain documents had to be written on stamped paper for them to be legal. In the 18th century you see ink stamps on newspapers too. A kind of tax.
Graham.
