It's a Winchester Model 1400 made for Sears. Depending on condition it is worth between 150-300. The Model 1400 was made from 1964-68. From @ 69-71 it was renamed the 1400 Mk II after a cosmetic checkering change and action release redesign. In 72 the "Mk II" marking was dropped and it was the 1400 until it was discontinued in 1994.
The rear of the barrel IS the chamber.
The chamber is the cavity in the rear-most end of the barrel where a cartridge is inserted, prior to firing. The chamber walls are usually significantly thicker than those of the rest of the barrel, as the chamber must withstand the majority of the pressure generated upon discharge of a cartridge.
Barrel marking, chamber cast.
Chamber or barrel.
The original factory barrel will have the name Winchester and the chambering of the gun stamped into it.The color is another tell, the color on the chamber should match the barrel exactly.Look at the joint between the barrel and the chamber, can you see any bare metal at the end of the barrel?A replacement will have to have had the threaded end of the barrel milled after it was blued to get it to fit right.
It is usually marked on the chamber or barrel.
if the gauge and chamber length are the same
It depends on the model. Rifles usually have one behind the barrel or under the barrel. Pistols may have one under the barrel or in the grip.
It should be marked on the barrel or chamber. If it is not, go to a gunsmith and ask for a chamber cast to be made.
If it is not stamped on the barrel or you do not trust the barrel markings, the only reliable way is to have a gunsmith make a casting of the chamber.
In the cartridge, which is placed at the back end of the barrel, in the chamber.
If it is not marked on the barrel, you should have a gunsmith measure the chamber length.