Thanks for the thoughts. No surprises there , then, so I'll stick with my trusty H & M Duette
It's still a mystery as to why, amidst all the rubbish "vintage" controllers on ebay, this type doesn't appear. Obviously that collector's shop has decided that it has the only surviving one in the world.
Actually the greater mystery is how this item came to be in friend's attic at all, as she says her ancient kiddies never had an electric train set. It came to my attention because it was in a bag of mangled scrap tin she gave me, that used to be the kids' clockwork 0 gauge Hornby (and yes, I did give the controller back to her, just in case it might be worth a million, though warning her that I thought even to get into double figures as a curiosity would be pushing it!)