HQ-WB used to develop rusty plenums and hence wet feet well before the end of the 70's. I'd bet at least 90% of the time it'll be rust when you get water leaks especially in Belmont/Holden luxury level. The worst offending spot in my experience is the front face of the plenum on either side where the plenum is very close to the firewall. In many cases the body dip or electrostatic red primer didn't get into that space and all it takes is some condensation. The right way to fix it is at resto time where you remove the whole cowl by drilling out all spowelds and replace either the whole plenum or pieces of it using a cut from a rust free donor. I've even seen people fix them using a whole firewall cut grafted in along the floor join and across the windscreen pillars.