The LW20 models used the short barrel extension and have the ejector riveted in the receiver like the 870, the LT20 models have the long barrel extension just like the 12 gauges, and the LTs have the scalloped out ejection port. K suffix is standard and U means Magnum on both the LW and LT models. The receivers within U and K are the same, but the barrel and action sleeve are different on the Magnums versus the standards. During the transition from the LW to the LT, there were some made that are neither fish nor fowl - I have seen an LW marked gun with the LT style barrel - so if looking for a barrel make sure the one you get matches what you have. An LW barrel can be made from and LT barrel. There were apparently some receiver issues with the LW, and that's why the LT was developed.
All 1100 20 gauges since 1977 are LT20s whether they specifically say it on the gun or not, and the barrels are interchangeable within them.
What are you wanting to do? Get another barrel for your gun, or get a new gun; I'm confused. They have made lots of LT20 Youth Models, BUT the later all black rougher finished versions are not as nice or as light as the polished versions. Mine with the 26" skeet barrel is lighter than a newer black youth gun. If you get any gloss finished LT 20 and add a blued youth barrel -or get the barrel cut and have tubes installed by a competent gunsmith - and a youth stock you will not be able to tell it from your gun in use. I have started out a lot of people including my kids with my LT20 Magnum with the Skeet barrel installed and a youth stock, and it works fine. A magnum barrel will not function reliably with lighter loads in my experience, but with any 2-3/4" barrel a magnum gun will work fine with anything up to light field loads. If you decide to shoot heavier 2-3/4" loads I would get a standard action sleeve or keep a real close eye on the buffer.
