Thanks Heinz. No, it hadn't occurred to me to search for the patent. Judging by when and where I found it I'd guess that it was already a long-forgotten artifact in my granddad's barn by the time the 1950's rolled around, but when I get the chance I'll try and see if there's anything to be found from the patent records.
I kind of hope it's not a nice old antique, then I can use it without worrying about it!
Thanks,
John