"It doesn't follow that any of these British cartridges were widely available anywhere in the United States"
Don't kid yourself.
Webley revolvers sold fairly well in the United States before World War I.
In addition, with Francis Bannerman snatching up surplus globally and selling it in the United States, these cartridges and revolvers, while probably not very common, were more than popular enough to support 40 or more years of cartridge production in this country.
Check this out for a sectioned view and headstamp on a UMC-made .450 revolver cartridge.
http://www.iaaforum.org/forum3/viewtopic.php?f=1&t=4058
And, lastly, remember who is to our North. The Canadians looked largely to the United Kingdom for their ideas on armaments, but they often looked to the United States for the ammunition for them.