The Iver Johnson "Champion" was first made in 1880. It had twist steel barrels, exposed hammer and a top lever to open it. The stock was walnut. It was called the Top Snap. The next version is the Side Snap which has a side lever to open it. There was also a Side Snap Hammerless. All were made from 1880 to 1900.
The Top Snap improved was made from 1901 to 1904.
The Champion Top Snap was marked "Champion" and manufactured from 1905 to 1908. It opened with a top lever, has a 28", 30", or 32" browned barrel, a rebounding hammer, and ejector. The receiver is case hardened and the stock is walnut.
The Champion Model 36 was made from 1909 to 1922. Its name was changed to Champion Single Barrel in 1923. They were made until approximately 1978. This version had blued steel barrels, ejectors, a case hardened receiver and walnut stock. It is probably the version you have. There were thousands made. There are no records of production dates on individual guns. If your grandfather purchased it used in the 40's, I would guess at a 1930's production date unless it has twist steel barrels. The values for your gun are:
excellent-$250, very good-$200, good-$150, fair-$75, poor-$25.
I hope this helps you,