Hi [ectropy]
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. I can't recall any other modifications done to it, I certainly didn't do - or oversee - any more. You have to remember that - at that time - none of us worked on 'all' Whos, it was purely another programme that the VisFX dealt with. You tended to get allocated your 'fair share' of them, but that was it. (OK, different today when you would - as a freelancer - usually be bought in to work over at least a season.)
Items were indeed bought from when Century 21 ceased business, (though I had nothing to do with this - it was done before I arrived there, though probably only 'just'?), including a lot of 'background' models. Some of which did appear in later Whos, the Draconian spaceship in The Frontier in Space is one that I know definitely WAS ex-C.21. (And virtually un- modified I recall?)
However - whether the basis of the SS also originated from there, that I am far less certain? Frankly I don't think it did - I think it was made 'from scratch' in the Department - although OK I fully admit I could be wrong? Possible Ian may remember, but we have the slight difficult in that he now lives in Bulgaria, and doesn't have email! (Well didn't as of recent attempts at contact.) I *may* have to contact him via snail- mail at some time in the near future, (about something else completely), so could ask him then - but I can't give you a time-scale for this.
Another point to remember is that these are 'working props', and changes are made all the time. A director wants 'this feature added', so it is - the fact that it was never there before, well....
I have this problem, (if it is as such), with K-9 where changes, some subtle, others not so, came and went all the time. It drives 'perfectionist' collectors / builders of replicas, mad, but sorry, that's the situation!
Cheers
PS - said I wasn't really going to be of any help....
mat