Try e-gunparts.com
The Browning medalist was the best by far.If you can find one,these were made from 1959-1974.
LR stands for Long Rifle which means the pistol uses a .22LR caliber bullet
You will have to call Browning to find out.
Highly unlikely that sn info is available. Highly unlikely that sn info is available.
Sterling Arms Corp of Gasport NY made the Model 283 Target 300 pistol in 1970-71. The 300L (light barrel) was made the same years.
On the back of the pistol you will find a long lever, slide it to one side and load in your BBs. Under the barrel you will find a thumb loader to load each shot into the firing chamber between each shot.
Try gun shops, guns hows, e-gunparts.com
Try e-gunparts.com
You may need to find a true gunsmith that can make one. Your pistol was imported 1962-68 by Tradewinds, Inc. Imported for so short a time, and that long ago, finding parts will be a matter of pure luck. Sorry-
Try http://www.gunpartscorp.com/
You can find Sterling trucks for sale using the dealer locator on the official Sterling Trucks website. You can also find them from personal sellers on the auto section of the eBay website.