The Standard Catalog of Smith & Wesson doesn't list many .38SPL variants but based on the ones I've seen for sale, I'm guessing that they've made a few runs of 686s in .38SPL over the years. I doubt the frames are different. From what I see on sites like GunBroker, they seldom sell for a premium price or at least not much of one. Some dealers start out asking big numbers for them but they don't draw any bids.

There are five for sale on GB right now and one on GunsAmerica.

Ed