You are buying the replacement blade shown in the pictures. This was taken from a model 4041, but it might actually fit a few others. It shows signs of use (the coffee mill it was taken from was several decades old), but nothing out of the ordinary. Great replacement part. 

In order to remove your old blade, you'll need to insert a small flat head screwdriver into the hole at the bottom of your unit and then carefully twist the old blade off counterclockwise. (if you don't have a small screwdriver, the bottom plate comes off with two philips screws and then you can access the middle flat-head screw easier). To install this one, simply screw it on clockwise and hand tighten (keep the same screwdriver in place). 

Ships fast!