The search continues - many thanks for the tip, KmPok; I've sent an email to the Packard Museum.
Incidentally, sources seem to agree that a total of 55,523 Packard Merlins were built, the last one rolling off the production line ( wherever that was....
) on September 20th, 1945.
Urqh - I'm quite sure that none were built in Canada. I think there's a little confusion there with Canadian-built Lancasters and Mosquitoes which all used Packard-built Merlins, shipped up, I assume, from Detroit....