Yep, most of that is accurate. But a few years back there were a few of the anomalies showing up at the bigger gun shows. Guys were offering up short barreled guns with square butts, and 4" guns with round butts. I have no idea if they were factory, or if someone got industrious and began swapping barrels to make conversation pieces. My guess is if you go seeking one or the other, you'll be disappointed, but don't be shocked if you stumble on one, either.
The old 66 short barrel guns were often called "ATF" specials. In the 70s, they carried as hefty a premium as the 29s did (and that was Dirty Harry era.) But then in the 80s and 90s, production caught up with demand. If you want a 2 1/2" gun, you can find them at the larger gun shows at pretty fair prices. Really about the same range as M60s. Just need to understand they're harder to conceal. The K cylinder is bigger around than the J. They're nice guns. Everybody should have at least one.