Earliest citation for truth density
The truth density $p$ is arguably the same concept as the bulge to 0 $b$ of a Boolean function as $b=1-2p\iff p=(1-b)/2$. The bulge leads to cleaner formulae in many cases though. The language of bulges was common at Bletchley Park. Do you count the General Report on TUNNY as a 1945 or a 2015 publication?