"It looks like these new 19s don’t have the forward lockup which means they are like the new 66s–ball detent lockup on the crane, which means a narrower ejector rod, which in turn means a full diameter forcing cone (without that flat cutout at 6:00 that was the Achilles’ heel of the older 19/66s)."
From a comment here:
http://www.thetruthaboutguns.com/2018/05/daniel-zimmerman/smith-wesson-reintroduces-the-model-19/
A round-butt, four-inch, pre-lock 66 has been on my wish list for a long time.
>"It looks like these new 19s don’t have the forward lockup which means they are like the new 66s–ball detent lockup on the crane, which means a narrower ejector rod, which in turn means a full diameter forcing cone (without that flat cutout at 6:00 that was the Achilles’ heel of the older 19/66s)."
From a comment here:
http://www.thetruthaboutguns.com/2018/05/daniel-zimmerman/smith-wesson-reintroduces-the-model-19/
A round-butt, four-inch, pre-lock 66 has been on my wish list for a long time.
Interesting question that I have no answer to. Maybe you can find a better link with more detailed info on that. Sorry.