I have owned 1100s and 11-87s since 1963 and have never run into such a situation. I don't mean to insult you, but are you sure the alignment stud is lined up with the groove in the barrel extension? Sometimes these are a bit of a tight fit. I just got a new Target Contour barrel to put on my 1963 Model 1100 a couple of weeks ago and it's a perfect fit. The number you ordered - 24638 - definitely should fit an 11-87. In fact I don't know of any 11-87 barrel that shouldn't fit, with the exception of the Super-Mag barrels. Does it say 3" chamber on the barrel? I have gotten a wrong barrel before, and it invariably came in a plain brown Remington barrel box with the number written on it in black magic marker. The ones with the green and yellow lables are always right. If you have checked everything else, I would call Remington. If it is a screw up, they will make it right.
The infamous "barrel lawsuit", was a class action cooked up by some lawyers to make money, and they did. Sometimes it is cheaper to settle than to fight in the good old U.S.A, even if you did nothing wrong. I have been there. As part of the settlement Remington agreed to move the rollmarking forward, they lightened it a little, and I thought they sent you a $50 check, if you filled out the paperwork. I told them to put my $200 worth in the ambulance chaser fund.
What could have happened... did.