Thank you, Henry, for a fascinating posting! (I wonder whether the Mr Majendie was related to the famous BOAC captain, or indeed the same man..?)
Contacting the companies is an idea I shall definitely have a go at, thanks!
I've gone down all manner of weird and wonderful avenues (British Library, Patent Office Library, now part of the Brit Library, and the Science Museum Library) and - incredibly- none of them has anything other than the same potted illustrated books one finds in Ian Allan bookshops... (not that there's anything wrong with them, but...) Discovered a couple of HS sales booklets, two in Zurich and one in Auckland believe it or not and had them shipped across to the Sofia Nat Library in August, only to find them full of sales type prattle and no hard info at all...