A question from the ignorant: How did they change the filters on those masks?

The filter canister is inside the mask around the cheeks.

Those filters were extremely difficult to change. As Tank 1812 states, they were accessed from the inside and a very tight fit.

Whoever designed that mask did not take into consideration that filters needed to be changed, and might need to be changed in a contaminated area. I dealt with the M17 series masks and could get the filters changed out pretty fast and there was no way I could hold my breath long enough to get it done. Thankfully they woke up and went to the M40 masks.

The photo guy looks like Joe? Reference "We Were Soldiers"