Since it has vent windows and what appears to be push button door handles, maybe a 52-53? If the door handles are pull down, maybe a bit earlier.
Some of the more knowledgeable users might be able to pin point it better. You can also take the serial number off the plate inside the drivers side door and decode it. There are a few resources online that will help.
http://www.stovebolt.com/techtips/VIN.htm