I put a new 3900 pack bag on my Unaweep frame yesterday and used it as an opportunity to break the pack down and get a weight of all the individual components.
Hipbelt (Large) 10.4oz
Harness 8.3oz
Frame 7oz
2" Extensions 0.6oz
old 4800 Pack Bag in X21RC 21.8oz (untrimmed)
new 3900 Pack Bag in X21RC 21.3oz (untrimmed) and 18.6oz after I trimmed it quite a bit.
I thought some people might be interested in a breakdown of all the components and am always looking for ways to save a few oz. I think the frame is about as light as you can get it. Most people could probably ditch the extensions, but I have a 21" torso so it fits me better with the extensions in place. The pack bag (especially after I trimmed the 3900 is pretty minimalist, so I am not sure much could be done there either. Making it out of a lighter fabric would help minimally, but the X21RC is already just 4.4oz per square yard and there probably isn't more than 2.5 yards of fabric in the pack, so you wouldn't save much.
The hipbelt and harness may have some weight savings available, particularly the hipbelt IMO but all in all it's a pretty basic simple setup. I have used lots of lighter packs, but I really like the way the Unaweep carries a load.