The best I can figure is that the 1977 K and the 1978 K seats are the same. The 1976 K and the K's from '79 onwards don't look like they would fit.
You might want to use the resources at
http://www.powersportspro.com/ and look a little further. (If you haven't used it yet, click on the I-don't-know-the-part-number thingy, register your bike, and get into the parts lists. You can back up, and change the year (and model) if you want. That way you can look at a lot of different seats from many years and models of bikes.
There's probably a better way, but this is the best way I can think of. I also use the free parts lists on
http://www.honda4fun.com/home.phpMaybe a more clever engineering type than I am could figure out a simple-to-make modification kit to expand seat interchangability....