The whole thing about Julian Jackson having a glass jaw is an overstatement IMO. He could be hit/stunned, but he wasn't outright put out.
I actually watched his bout with Thomas Tate recently, and he took some hard shots from Tate in the fight.
McCallum was the only guy to stop Jackson in his prime, and Mike was a heavy hitter. And Jackson was still on his feet when the fight was stopped. I also don't put too much stock in the McClellan knockouts because
A) G-Man had one hitter quitter power
B) By the time Julian got him, he was having alot of problems with his vision due to his retina damage, and was on the downside of his career anyway after 50+ fight.
By the 2nd fight, he was basically a spent fighter with a deteriorated chin. Counting that is like counting SLR's KO to Camacho.
Basically, I'm not saying Jackson had an iron chin, but it wasn't glass like everybody has made it out to be.