Here’s a nice article on the IJ revolvers.
http://www.armchairgunshow.com/pocketDA-info.html
It was written by Jim Supica who’s the curator of the NRA firearms museum, so I guess it’s credible. :)
While the article is great it does NOT specify when the Hammer block was introduced by Iver J. I believe you when you say they were introduced at the turn of the century but I saw nothing in the article to indicate that was the case. Iver J was a greatly underrated manufacture of guns. They were great firearms. Thanks for the article, I enjoyed it greatly.