They should have been able to tell you one more thing.
If the serial number ends in M, it's a Magnum receiver. This means you can shoot up to 3" lead shot shells in it, as well as 3" steel and 2 3/4" lead, though you do have to change barrels for different types/sizes of shells.
If it doesn't end in M, then you can shoot 3" steel shot shells in a "steel shot barrel", still available new from Remington and others, and 2 3/4" lead shot shells in standard 2 3/4" barrels.
I have a gun that looks just like that, except mine's got field wear. Yours looks near-new! Unless it says Magnum on the side of the receiver, it's the standard field model. Others at the time would have been Skeet, Special Field (English stock and short barrel), etc. It isn't one of those.
Steve is right: I have a fixed-choke Modified 28" plain barrel and it patterns really nicely. Breaks clays and brings down birds at longer ranges than it should.
Assuming the barrel has not been messed with, the standard for the plain barrels with fixed chokes was, AFAIK, 26" IC, 28" M, and 30" F. This makes sense, if you consider swing, range and pattern size for different uses. You can guess what your barrel is, probably, by the length. I can't see anywhere on my 28" barrel that indicates Modified, but 2 3/4" is stamped on the side in big letters. I have a 3" Full barrel as well (I have the Magnum model from 1971); it's 30" and I don't think it's marked for choke either.
Nice gun!
Don't let it go, unless you want to sell it to me, of course. Remington produced a world-class gun then, with good QC, and the 1100 remains a world-class design.