Last picture shows where a "fill probe" is inserted. Like this one in the link.
https://www.pyramydair.com/s/a/Air_Venturi_Replacement_Probe_Fits_Select_Kral_Arms_Evanix_Hatsan_Guns/3985
https://www.pyramydair.com/s/a/Air_Venturi_Replacement_Probe_Fits_Select_Kral_Arms_Evanix_Hatsan_Guns/3985
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