On the first 39s, the bottom part of the dust cover was open. So, the bottom part of the bushing had the guide rod and recoil spring pushing against it. This caused some rather serious tension on the bushing and the bushing would eventually break and the guide rod and spring and the bottom of the bushing would be launched downrange rendering the gun, obviously, disabled. This was found due to the use of the Illinois State Police over many years as their service sidearm.
The solution was to close the bottom of the dust cover, thicken the bushing and this would allow the bushing to be cut so to let the recoil guide rod pass through and not have any possibility of breaking and losing the rod, spring and bottom part of the bushing launched downrange.
This redesign made the bushing stronger and left the possibility of failure a near impossibility at this point of the gun.