The 1950s PC my mother-in-law had was the same as the one I grew up around. It had a hard rubber plug in the upper lid with a metal dumbbell valve plug in the middle of that rubber disc. That metal valve would bleed a little pressure when the cooker was a the desired pressure but if the pressure was way over the top, the rubber plub could blow out and pressure was relieved straight up. In the case of my story, to the ceiling.
In my mother’s pressure cooker ordeal, the kitchen ceiling was not spared.