Imperial Pale Ale?

Well, without doing any research there's this ad came to mind - from my files:

Granted, it could be said that it's the American retailer's usage, not the UK brewer's.

There's no mention of "Imperial pale ale" in Wahl & Henius that I can find after a quick search and, of course and as noted, US stock ales of that era would have been long aged before bottling (9 months to 2 years), so any relationship to modern "Imperial pale ales" would be in name primarily. In the US post-Repeal era, the term "stock ale" was apparently used for more beers than was "India pale ale", but the terms were also used somewhat interchangeably for highly hopped, long-aged strong ales.

For example, from a 1935 P. Ballantine & Sons ad for their Ballantine's India Pale Ale:

"It is a real 'stock ale' in that it is not only fully and carefully aged but it is, in addition, 'dry-hopped'---"​

Of course, "beer style/type" terminology usage was much looser in that era, particularity in the US.

I'm not sure if Mosher is saying that today's beer labeled "Imperial pale ale" are similar to the US stock ales of the past (his use of "is" implies that's the case) but there are a few examples of historic claims in Mosher's books that I've questioned, and his books aren't well documented as far a specific citations or footnotes to original source material.

And, to throw a monkey wrench into the discussion of the US usage of the "Imperial" term- an ad for John Taylor & Sons, the famous Albany ale brewer and the once among the largest US brewers in the era before the dominance of lager beer in the US: