I contacted Mat Irvine, formerly of the BBC Visual Effects Department, in an attempt to obtain more info on the classic sonic screwdriver.
Quate from Mat Irvine:
Sorry, I'm probably not going to be of much use here. The only contact I recall ever really having with that particular incarnation of the Sonic Screwdriver, was (I think?) during the Pertwee era. Then, as one of the new Assistants in the Visual Effects Department, I added a button magnet (as you surmise) to the top. (Presumably as The Doctor has to pick up something ferrous - though at this stage, don't ask me what!) Apart from that, although the SS was part of the standard equipment we would have taken into any Who studio, that was about it.