I am chimming in on this late but I may be able to add to the info. I have a 48 GMC with the center rear trunion mount with the corner rubbers and found a rebuild kit at Classic Parts. Everything fit properly but the rubber pads are 1/4" thicker than the originals. At first I thought the old ones had taken a 1/4" set but I don't think so. The way it was designed to work would require the cab to sit on the pads taking the weight and absorbing shock. I put a 1/4" shim under the center trunion mount so the system looks perfect except its not as designed. Shop Shark is correct in his info. GM redesigned the mount putting a trunion at each rear corner. The stress of the cab flexing 1/4" before it touched the corner rubber was tearing up the surrounding sheet metal at the center rear of the cab where the trunnion mounts. Mine is so destroyed by fatigue and a failed weld and further buggered by the woodpeckers that owned it over the years that I cannot make out the original sheet metal enclosure that covered the trunnion from the inside of the cab. If anyone has a picture os sound sheet metal in a 47-48, I would love to see it.
