In message , Christian
McArdle writes
Only real way would be to lift a floorboard upstairs.

Remember that many, many original lath ceilings will have been augmented by
plasterboard underneath, as it is an acceptable and time (and mess) saving
alternative to replacement.

yep, that's exactly what someone would find in this house.

poking around from underneath is the way to go here.
--
Chris French, Leeds