Twelve gauge was never offered with the short chamber; they were always 2 3/4 inch, or 3-inch in the case of Heavy Duck models. Many of the earlier short-chambered 16 gauges were lengthened by the factory or private gunsmiths. (This information is from Madis' book on the Model 12.)

That's a sweet gun in that gauge with the solid rib, and if you've paid any attention to prices for Model 12s in that gauge and barrel configuration, the price doesn't sound too bad. It may be worthwhile getting a chamber gauge from Brownells; mine has come in handy.