Yours look almost identical at first glance. Sounds like a great way to go after I learn on the 617 .22.
The 617 is a "K" frame and the 686 a slightly larger "L" frame, although the grip size is the same on both. I actually shoot three revolvers that are very similar: the Model 617 .22 and Model 686 .38/.357, as well as a Model 16-4 .32 Magnum (also a "K" frame, and will shoot .32 S&W Long and .32 Magnum cartridges).
By the way, this is not a new concept. In the late 40's, Smith & Wesson manufactured the K-22, K-32 and K-38 Masterpiece revolvers on the same frame with the same weight for practice purposes. In fact, the box lids read something like "The K-22 Masterpiece, companion gun to the K-32 and K-38 Masterpieces".
Clockwise from bottom: 16-4, 686, 617.