Steve, I have a late mk3 in my garage with twisty handles, and my old one (sold when Gill was too pregnant o use the car for work, 24 years ago!)also had twisty handles. Not sure when the changeover happened, around 68 I reckon?
The earlier cars indeed used hoodsticks, but the screen frames are the same, the capping was different with the lip bit. And obviously no holes for any clips/handles etc, but they can be transferred across with the aid of a mig to make good. Likewise GT6/TR.
25 years ago mint imported frames were £60-80..... rather different now!
The daft thing is, I HAD a couple of screen frames, one from a mkIII and one from a GT6mkI and I can't, for the life of me, remember where they went, i'm sure they weren't sold and they were much too good to scrap SO WHERE ARE THEY????? I do still have a badly twisted, accident damaged TR6 one which might yeild a few repair sections, but the 2 complete ones would see me sitting on a goldmine! I think the change would have been later than 68, more like 1970. I had a 68 MkIII with the old style latches SPA 632F It's theoretically possible to fit a MKIV frame to a mk III and again, transfer the latch striker plates, the holes would be in the screen frame for them because they are, IIRC, the holes the hardtop bolts go through! Or am I talking cobblers again?
I also still have a GC MKII hoodstick in wedgewood blue if anyone is interested! I really should have a good clearout and get some of this stuff moved on to where it can do more good!