These look the same as today's air packs with the face mask and the cylinder worn on the back. By the mid 1970's we had a variety of air packs made by different manufactures and different models of the same manufacture. We had Scott Air-Pak, Scott II, Scott IIA, Scott 2.2, MSA, and Surivair air packs. Demand is air supplied only when the user takes a breath. If there is a leak in the seal of the mask the user could inhale toxic gases.
Positive pressure provides the user with a slight positive pressure of air in the mask at times.
If there is a leak the air will leak out of the mask instead of letting toxic gases in the mask. Only positive pressure air packs are permit to be used in the fire service today. With the exception of the Scott 2.2 air packs which had a special fail open regulator that eliminated the need for an emergency bypass valve. The original air packs used steel air cylinders or bottles. In order to cut the weight of the air packs the manufactures were able to make aluminum air cylinders.