To add a comment, these changes were made to reduce production costs.
There is no need to pin a threaded barrel to a frame or receiver; it should be screwed in tight anyway.
Further, if you have ever taken a pinned S&W barrel off, the groove through the threaded portion is bigger than the pin. Barrel and pin don't even touch. The pin doesn't do anything unless the barrel's loose, and then it would let the barrel turn a little but not come off.
Second, recessed chambers came with the 45,000 PSI .357 in 1935 with the claim of greater strength. MAYBE if you loaded old balloon-head .38 cases with very thin brass at the head to 45,000 PSI the recessed chamber would be a benefit, but with the brass being made today it's unnecessary. The .500 S&W operates just fine at 60,000 PSI and non-recessed chambers.
JR