Phil- I emailed you Mike- buickV8Mike- Dave and Phil are talking about early 1970 models that were built in 1969...NOT 1969 Buick GS and Skylarks. The ONLY rubber parts on the rear slide into the corners of the bumper. These rubber "spacers"/ blocks that are being discussed are for 1970-1972's.
So just to add on to this thread. I pulled these off my low mile GSX about 2 days ago. I don't believe the rear bumper has ever been off my GSX so add this to the mystery of these parts