The "plug" in your 1200 is just a stick slid down inside of the magazine spring to limit the number of rounds that will fit in the tube. Keep in mind that there is a big long spring inside that magazine tube and if you're not careful, when you unscrew the cap at the muzzle end of the magazine tube, it's going to go flying across the room. Once the cap is off, remove the wooden (or plastic) "stick" that will be tucked inside the spring. Push the spring back in and put the cap back on. Save the plug, you may want to put it back some day.
Not recommended.
100-300 or so
Proofhouse.com has Winchester serial number data.
The best that I can do for you is say that the Winchester model 1200 shotgun was made from 1964-1981.You may want to check with winchester home page and see if there is any help for you.
Everything from squirrels to deer.
750 to 1200
The Winchester model 120 ranger was a economy version of the Winchester model 1200 shotgun.the model 1200 was produced during 1964-1981,and the ranger was also produced during this time.
No, there is only room for 2 shells, ever. You cannot increase it's capacity.
100-300 or so
If you don't have a owner's manual, find a gunsmith and ask for help.
Yes.
20% of 1200= 20% * 1200= 0.2 * 1200= 240240