This is a Vintage Corbin brass flying eagle two combination dial post office box door. Could possibly date back to the 1930s and was still being used in the 1950s or later. The picture showing the dial settings represents the actual combination, and I will include a sketch of how to determine  the combination setting and also describe how to change the combination if you wish to. This particular sizing of door is a little larger than another popular style. This one measures approximately 6 1/4" x 5 1/2".