Functionally, the No. 12 is the predecessor to the No. 20. They use the same motor. Sterling's corporate records were hit and miss so there's no certain way of determining when anything was introduced except the Oscillite. Somewhere I have a photocopy of their Corporate Record book covering about thirty years, beginning with their relocation to Rochester. You'd think such a document would be a treasure trove but alas, there's precious little useful information.
The No. 12 is a beautiful instrument. Early versions were nickel plated, later versions chrome plated. Compared to a more serious instrument, however, a No. 12 is more of a relic.
Normally I would advise you not to worry about the voltage of the motor as 6 volt motors generally perform well at 12 volts. With Sterling products I'm only willing to say that about the No. 30. The armature on smaller Sterling sirens is just too flimsy to safely overspeed.
Keep the faith, I think a Super Chief is in your not-too-distant future....