Yes, in an old Kodak application note, it says:
"There are two basic types of ISO: saturation-based and noise-based.
The saturation-based ISO is also referred to as the ‘base ISO'.
from: kronometric.org/phot/exp/ISOiawKodak.pdf
It's an old Note, the other methods introduced later by ISO (SOS, REI) are not mentioned at all.
Indeed so, Dave. Thus it was that Canon was allowed to reduce the headroom afforded by saturation-based, according to this well-respected
Canonista:
http://kronometric.org/phot/exp/Exposure_Calibration.pdf