The serial number should be on the bottom of the grip part of the frame. If you have target grip which cover the frame you need to take them off. If you open the cylinder the area the crane latches into should have the model number. If it does not you may have a pre-27. If it is the gun should have a bright blue finish and the top of the gun from the rear sight to the front sight should have a fine checkered pattern. If it has grooves it is not a 27.