More than for the period, the correct units for the nation are metric, which most of Europe was using then for decades.
You are probably reading an american translation whose translator decided to translate units to imperial system.
That's an issue in many books, sometimes translators or editors choose to change the units from the original to local, but it's historically unaccurate and misinforming. I've had it happen both ways, some american or british books (uncorrectly) translate metric to imperial, but I also have some spanish books where they've translated imperial info into metric. For me, both cases are deeply wrong.
Sometimes (as I think they should, if they want to change anything) they note the data in both systems, so inexpert readers can get an idea of the data they are reading.
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