Good question, I tried to do it in the center but after I looked at picture of real Combine, it was near that gas mask thingy

also dont forget that these people are humans changed into transhumans (no im not talking about genders) and probably after all of the changing human into that thing, some things may change like eye position etc. since they have added some strange things into brain which changed everything like their Tactical Solutions, shooting Skills, communication Skills and remove human mind etc. Which made them forget about how they became Combine Soldier and actually become friendly to the fellow combines

Also take an look to the “Stalker” that isn’t just an anomaly but an transhuman which is being in the state of being created like his Combine armor and other. There were cases that Stalkers became hostile anomalies and attacked everyone

Thats what i know
(idk If its real but i am reading Combine wiki)