Good luck with this!
I advertised in various aviation magazines for many months before I eventually got a response. The owner hand-delivered it to me in Hong Kong (he flew for Virgin Atlantic while I was then with Cathay), he told me he'd removed it from an EE Lightning that was about to be struck off charge. It came with a backing plate but once this was removed it fitted the Chipmunk's brackets perfectly.
Subsequently, ex-RAF Chipmunk drivers have told me that the chromed stopwatch was much more common - but I like mine!