The basic difference I see between the 1200 and the 1300 is the ejector retainer; on the 1200 there is a pin hole on the Left side-upper rear corner, the 1300 does not-it has an inside the left side retainer at fwd lower.
So by the pin hole on the left side I'd say my shotgun is a 1200.
Maybe they came up with the 120 Ranger originally to use up 1200 parts.
Be nice if they marked the reciever with the model number in case one needs parts!!!