I was given a 20 gauge Remington 1100 slug gun with a 21" non-rifled slug barrel from my dad about 5 years ago, and I wanted to put a new stock and scope mount on it, but I ran into some difficulties. I wanted to give everyone some info so they don't have the same problems.
The problems centered around the fact that the gun was a pre-1976 1100 20 gauge. Apparently, Remington decided to build their first 20 gauge 1100s on existing 12 gauge platforms. Because of this, the receiver and magazine are actually from an 1100 12 gauge, and not a 20. I had trouble finding a stock to fit until I realized that most aftermarket 1100 12 gauge accessories will fit on the pre-1976 receiver.
Anyways, just wanted to let anyone having the same problem know. Here are some after pictures. It has a Ram-Line synthetic stock and Bushnell trophy 1x red dot sight on it. Shoots 1.5" groups at 50 yds with rifled slugs.