I don't think this is quite true. Certainly many of the 'H' powders are renamed ADI stuff, made in Australia. There is a list on either the Hodgdon or ADI web site (or maybe both) giving the details. However, as I understand, the IMR powders are made in Canada, by a company bought by Hodgdon a few years ago, and they are not quite the same as their H counterparts.
Hodgdon's on-line reloading data shows loads for both IMR and H powders of the same name - or number - and they are not identical.
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