I was watching the story Robot (first episode with Tom Baker) and noticed that the Sonic Screwdriver that the Doctor (Tom Baker) used in this story had a removable head. I don't recall this detail being discussed in any of the threads about the Sonic Screwdriver. It does make sense that the Sonic Screwdriver has an extending head in the story "Genesis of the Daleks" which was about three stories after Robot if I recall correctly.