When S&W makes a batch of guns destined to be commemoratives or otherwise unique, they make more of the base gun to insure they have enough to fill the order if some of them are damaged or otherwise flawed during production. The exess unmarked guns are then sold through their normal distribution network.
In the case of the 'Twelve Revolvers,' the original plan was to manufacture 500 sets of 12. The demand for the sets was lower than thought, and there were a pretty fair number of guns made but sold unmarked, like yours, or as marked single guns.
You have a very desirable gun there.