The "Classic" that preceded the "21" had blade steel that was almost exclusively S30V. The 21 started with S30V steel and is now almost exclusively S35V. The handles are almost identical except for some different angles grinds along the edges. The back-spacers on the later 21's are different than the Classic. More recent 21's and most new Sebbies have different washers between the blade and handles the previous models. Newer 21 models are stamped "Idaho Made" and the screws that hold everything together have hex fitments on both sides of the handle. Older 21 and Classic models had a different pin setup and had hex fitments on only one side. There are other differences, but it gets fairly prosaic from there on out. Personally, I like a good "Classic" best, but I own several 21's and they all function the same to me. Fit seems every bit as tight and extremely high quality on Classics and 21's. I have no problem sharpening either S30V or S35V on a standard Sharpmaker, but there are massive essays about which steel is better, easier to sharpen, more chip resistant, etc... Whatever... Buy what you like I'd say. They are all pretty damn awesome.