[steely]

The manuals may have been tailored for the country for which they were written.
They'd never bother to revise them just to switch measurement standards, they'd go nuts.
Revisions were made to fix or update, not for unit conversions.
The metric or SI standard is pretty much worldwide except for a lot (if not most) English speaking countries where they use English units.
In the 60's or early 70's there was talk of the US switching to metric to comply with the SI standard, but it never took hold.