You need a professional appraisal
The one I have has a 20 inch barrel.
If your Savage model 720 riot shotgun has between 60%-80% of its original finish remaining,then it will be valued at between 600-925 dollars.
Assuming a standard model with 90-95% original finish (unless you you have a short barreled riot gun, which is unlikely), it should retail for $155-$165.
100-400 USD
Model 12 serial number 906770 was manufactured in April of 1941, and yes, they were made with a 20-inch barrel (the Riot Gun and Trench Gun).
It depends on the condition of the gun.
The value of any gun is based on the exact make, model and condition. There are several different grades of the Model 12- values could be around $200, or $1,000. You will really need a hands-on review of your shotgun. Take it to a good gun shop or a gun show.
leave off the 19 on the serial number as this is my 1957 manufactured model 12 riot
You can't value anything with just a serial number. Who made it? What condition is it in? Is it a hand gun or a long gun?
Try the Blue Book of Gun Values
Remington didn't make a Model 12-2 but Model 12-A-F & and CS. But the value is GREATLY dependent on condition without that it's anywhere from a 200-1,200 gun NIB.
100 to multiple thousands depending on EXACTLY what you have and its condition.