This has been covered a few times, and you can search through for more detailed info...
But they are located, one at each corner of the vert, a large sealed cylinder containing a weight and spring mechanism and filled with Fluid do help eliminate excess vibrations because of the convertibles tendancy for flex and vibration.
These were added to help eliminate these vibrations, and to help give the car an over all smooth ride.
Many times these will have been taken out of the cars by previous owners for one reason or another. They are getting harder and harder to find. 67's are different from 68's, and I think the 68's & 69's are similar, but may have subtle differences?
They weigh about 22lbs each, so the weight factor may be why many would have taken them out. Some can feel the difference, while others say there is little difference in ride quality?
I only have the two rear one in mine, and feel it rides just fine. But have never had the fronts to feel the difference?