There is some additional information here
http://67-72chevytrucks.com/vboard/s...d.php?t=552004
The cab shackle mount functions and looks a bit like a leaf spring shackle mount. The shackle mount was introduced in 1949? Replacing the center mount design. It bolts to the outside of the frame near the back of the cab and extends up to the cab, bolting in place.