"I guess pinning the barrel wasn't that big a deal to them!"

Maybe they didn't want the change to be noticed!
S&W has been inconsistent with model and dash changes over the decades. When the M15 was modified to fire magnums, the model number was changed to M19. When the M60 was modified in the same way in the late 1990's, the model number stayed the same, only the dash number changed to dash 9 and 10.

Best and Happy Boxing Day,
Rick