But not so vintage, this one is actually made in Korea. Classic straight foam shoulder straps with no contouring, and a simple rectangular backpanel. As stiff as the backpanel appears in profile I assume it may be one of the foam frame sheets with integrated metal stays.
Karrimor is one of only a few makers to put a hanging belt on an internal framed pack. This particular one is attached at both sides and bottom and appears to have a good bit of adjustment for ride height.
A wide attachment area provides stability, but it can't be too securely anchored or you can prevent the belt from contouring the body properly.