So it looks like both those nipples are air outlets?, so the pistons alternate back and forth, where is the air inlet?... That drawing isn't very clear
***EDIT***
This model has only one air outlet, but the url still refers to it as a double-outlets ( Chinese nomenclature

) neither models are clear where the air inlet is, unless its that unmarked hole next to one of the nipples... not that it matters, I just like to know where the air inlet is.
https://www.maxcleversz.com/html_pro...r-pump-93.html