There were several versions from 4000 up to 6500. I cannot find a decent review on the 6500.
Kelty have now a Red Cloud that is 6500 ci, 6 lbs, but of course I have no idea what load it will hold.
I would be happy to be able to walk 1 hour with 100 lbs on my shoulders. ( not that I would want to do it)
This reminds me that my grandfather used to carry that in sugar and coffee just after WW2 for his own private import business ( from Switzerland to Italy. About 6 hours with a 3000' vertical gain both sides and at night)

He had an "external frame"
Franco