The information I have is that he (William Martin-Hurst ?) was actually walking around the Buick R&D dept. on other business at the time, he spotted one of the engines (obviously abandoned) and asked about it, he was told it was out of production due to being too small for current needs. After a quick measure, verbal agreement was made there and then for Rover to buy the rights to it.
Nothing what so ever to do with any 'Motor Show' and I don't think there was any boat yard connection either, but Dave could well be correct in why the people from Rover were in the USA.