Oh, forgot to mention in the last message.
If you take a look at the episode ROBOT, (which is the first story with Tom Baker as the Doctor), in the last episode, the Doctor uses his SS to try and open the bunker door and stop Think Tank. When he begins using the SS, it does not have the "rounded bullet head" on it. Its only later, when he added the "rounded bullet head", he explains that he turned the SS into a sonic lance. Im assuming that after it was added, the producers decided to keep it on for the rest of the show. Mind you, it does not explain the "bullet head" on Pertwee's SS, but I thought it was an interesting change that was part of the story...
-Skyler101