Dick, a fifties vintage Custom Built gun would have a rounded frame and a different checkering pattern. Paul's gun is most likely a special order 20 Gauge Magnum model, which was a standard catalog item, not a custom order gun. The vent rib and 28" barrels would be special order features of the standard catalog gun. I would be very surprised if it has engine turning on the flats and a rib inscription. However, on occasion, Winchester would build a gun on a standard non rounded frame with Custom Built features in the period after the Deluxe Grades were discontinued. I think the serial number is way too high to be a Deluxe marked gun.
