I think what you mean, is that he had a 300lb sandbag that he used to chuck around for strength training.
The only footage I've seen (barely) seen of Jeffries where he doesn't look like the most overrated fighter of all time is the second Ruhlin bout and that's a big leap of faith as you can barely see anything .. lets throw out the Johnson fight event hough he didn't show one minute of anything ... the existing footage Sharkey footage is horrible .. almost inexcusable ... by pick Norton loses to the Jeffries legend but destroys the guy I think I've seen.
Jefferies was one tough SOB...and his stamina was miles ahead of Ernie Shavers...he always gassed early...and never had a KO past the 7th round!
It was not analogous to the Frazier or Ali that you mention because both of those were in steep decline when they had previously retired. Jeffries was peak upon his retirement and had a short career, fighting many times against smaller opponents. He simply didn't absorb the damage that Muhammad and Joe absorbed. He was closer to the Vitali that came back against Peter.