Looks very much like the American framed packs used on their first successful ascent of Everest. Designed and built for Recreation Equipment in Seattle, WA.
I have one, it's taller with 4 outside pockets. Has lasted me around the world.
The best part is that the bag is rigged to be easily detachable for carrying loads, even boxes, from one camp up to the next.
Pro's: for loads, you're going to love it.
Con's: the back and waist bands stretch when they get wet (sweat or rain/snow).
Despite the covers, you really better dope up the pocket zippers so they can't freeze.
Not sure what the bands are on my old pack. Could even be cotton.
The old style nylon climbing rope is designed to stretch 40%. I never had to test mine in a fall.
What does your pack use for pocket closures? Velcro?
#18 nylon cord stretches also. That's how to dry whip a steel blade onto an elbow and D adze handles.
I use the #18 tarred nylon seine cord which does have a noticible stretch. Have to take advantage of that.
I did google now and most show the pack going from the top of the frame to about 15 cm from the lower ledge.
I had a Al framed Fjällräven ( broke the frame, ripped bag) then an al framed Haglöfs backpack ( frame broke) that were basically clones. All went up to the top of the frame. You could buy top extensions so you could add another pack ( sleeping bag, tent) above it.
I would not treat it as it is an old fabric, do not know how waterproof it will get. If not 100% waterproof - wasted effort and money.