As minconrevo stated above, they are two different size frames. The smaller J Frame uses a coil mainspring and the K frame uses a leaf spring. They are similar in design over-all, but parts are for the most part not interchangeable.
I've got a few examples of both and while the J Frame is handier to tote around on a camping trip - the K Frame M18 is a superior revolver - at least to me. Easier to grip, shoot and smoother overall.